Story highlights Pilots' computers show nothing to indicate sudden deviation was preplanned

Every one of the 239 people on board is both a possible victim and suspect

The plane's pilots have become one obvious area of focus

A 29-year-old Malaysian civil aviation engineer is also under the spotlight

Until authorities know what happened to Malaysia Airlines Flight 370 , they'll look for clues in the histories of everyone on board.

The cruel reality is that every one of the 239 people on board is both a possible victim and a possible suspect -- until proved otherwise.

Already, some passengers and the pilots have fallen under increased scrutiny, and more are likely to come into focus as the search for answers continues.

"You have to look at everybody that got onto that plane," Bill Gavin, former assistant director of the FBI in New York, told CNN's "The Lead with Jake Tapper" on Monday.

"You can start peeling the onion there by eliminating some of the people immediately -- you know, like children, and maybe very elderly people, or infirmed people. You might be able to eliminate those folks.

Photos: The search for MH370 Photos: The search for MH370 Two years after Malaysia Airlines Flight 370 went missing, a relative of one of the passengers burns incense in Beijing on March 8, 2016. Flight 370 vanished on March 8, 2014, as it flew from Kuala Lumpur, Malaysia, to Beijing. There were 239 people on board. Hide Caption 1 of 43 Photos: The search for MH370 On July 29, police carry a piece of debris on Reunion Island, a French territory in the Indian Ocean. A week later, authorities confirmed that the debris was from the missing flight. Hide Caption 2 of 43 Photos: The search for MH370 Staff members with the Australian Transport Safety Bureau examine a piece of aircraft debris at their laboratory in Canberra, Australia, on July 20. The flap was found in June by residents on Pemba Island off the coast of Tanzania, and officials had said it was highly likely to have come from Flight 370. Experts at the Australian Transport Safety Bureau, which is heading up the search for the plane, confirmed that the part was indeed from the missing aircraft. Hide Caption 3 of 43 Photos: The search for MH370 In late February, American tourist Blaine Gibson found a piece of plane debris off Mozambique, a discovery that renewed hope of solving the mystery of the missing flight. The piece measured 35 inches by 22 inches. A U.S. official said it was likely the wreckage came from a Boeing 777, which MH370 was. Hide Caption 4 of 43 Photos: The search for MH370 Relatives of the flight's passengers console each other outside the Malaysia Airlines office in Subang, Malaysia, on February 12, 2015. Protesters had demanded that the airline withdraw the statement that all 239 people aboard the plane were dead. Hide Caption 5 of 43 Photos: The search for MH370 A police officer watches a couple cry outside the airline's office building in Beijing after officials refused to meet with them on June 11, 2014. The couple's son was on the plane. Hide Caption 6 of 43 Photos: The search for MH370 Members of the media scramble to speak with Azharuddin Abdul Rahman, director general of Malaysia's Civil Aviation Department, at a hotel in Kuala Lumpur, Malaysia, on May 27, 2014. Data from communications between satellites and the missing flight was released the day before, more than two months after relatives of passengers said they requested it be made public. Hide Caption 7 of 43 Photos: The search for MH370 Operators aboard the Australian ship Ocean Shield move Bluefin-21, the U.S. Navy's autonomous underwater vehicle, into position to search for the jet on April 14, 2014. Hide Caption 8 of 43 Photos: The search for MH370 A member of the Royal New Zealand Air Force looks out of a window while searching for debris off the coast of western Australia on April 13, 2014. Hide Caption 9 of 43 Photos: The search for MH370 The HMS Echo, a vessel with the British Roya; Navy, moves through the waters of the southern Indian Ocean on April 12, 2014. Hide Caption 10 of 43 Photos: The search for MH370 A Royal Australian Air Force AP-3C Orion, on a mission to drop sonar buoys to assist in the search, flies past the Australian vessel Ocean Shield on April 9, 2014. Hide Caption 11 of 43 Photos: The search for MH370 A relative of a missing passenger cries at a vigil in Beijing on April 8, 2014. Hide Caption 12 of 43 Photos: The search for MH370 Australian Defense Force divers scan the water for debris in the southern Indian Ocean on April 7, 2014. Hide Caption 13 of 43 Photos: The search for MH370 A towed pinger locator is readied to be deployed off the deck of the Australian vessel Ocean Shield on April 7, 2014. Hide Caption 14 of 43 Photos: The search for MH370 A member of the Royal New Zealand Air Force looks at a flare in the Indian Ocean during search operations on April 4, 2014. Hide Caption 15 of 43 Photos: The search for MH370 On March 30, 2014, a woman in Kuala Lumpur prepares for an event in honor of those aboard Flight 370. Hide Caption 16 of 43 Photos: The search for MH370 The sole representative for the families of Flight 370 passengers leaves a conference at a Beijing hotel on March 28, 2014, after other relatives left en masse to protest the Malaysian government's response to their questions. Hide Caption 17 of 43 Photos: The search for MH370 A member of the Royal Australian Air Force is silhouetted against the southern Indian Ocean during the search for the missing jet on March 27, 2014. Hide Caption 18 of 43 Photos: The search for MH370 Flight Lt. Jayson Nichols looks at a map aboard a Royal Australian Air Force aircraft during a search on March 27, 2014. Hide Caption 19 of 43 Photos: The search for MH370 People in Kuala Lumpur light candles during a ceremony held for the missing flight's passengers on March 27, 2014. Hide Caption 20 of 43 Photos: The search for MH370 Malaysian Prime Minister Najib Razak, center, delivers a statement about the flight on March 24, 2014. Razak's announcement came after the airline sent a text message to relatives saying it "deeply regrets that we have to assume beyond any reasonable doubt that MH 370 has been lost and that none of those onboard survived." Hide Caption 21 of 43 Photos: The search for MH370 Grieving relatives of missing passengers leave a hotel in Beijing on March 24, 2014. Hide Caption 22 of 43 Photos: The search for MH370 A passenger views a weather map in the departures terminal of Kuala Lumpur International Airport on March 22, 2014. Hide Caption 23 of 43 Photos: The search for MH370 A Chinese satellite captured this image, released on March 22, 2014, of a floating object in the Indian Ocean, according to China's State Administration of Science. It was a possible lead in the search for the missing plane. Surveillance planes were looking for two objects spotted by satellite imagery in remote, treacherous waters more than 1,400 miles from the west coast of Australia. Hide Caption 24 of 43 Photos: The search for MH370 Satellite imagery provided by the Australian Maritime Safety Authority on March 20, 2014, showed debris in the southern Indian Ocean that could have been from Flight 370. The announcement by Australian officials raised hopes of a breakthrough in the frustrating search. Hide Caption 25 of 43 Photos: The search for MH370 Another satellite shot provided by the Australian Maritime Safety Authority shows possible debris from the flight. Hide Caption 26 of 43 Photos: The search for MH370 A distraught relative of a missing passenger breaks down while talking to reporters at Kuala Lumpur International Airport on March 19, 2014. Hide Caption 27 of 43 Photos: The search for MH370 On March 18, 2014, a relative of a missing passenger tells reporters in Beijing about a hunger strike to protest authorities' handling of information about the missing jet. Hide Caption 28 of 43 Photos: The search for MH370 U.S. Navy crew members assist in search-and-rescue operations in the Indian Ocean on March 16, 2014. Hide Caption 29 of 43 Photos: The search for MH370 Members of the Chinese navy continue search operations on March 13, 2014. After starting in the sea between Malaysia and Vietnam, the plane's last confirmed location, search efforts expanded west into the Indian Ocean. Hide Caption 30 of 43 Photos: The search for MH370 A Vietnamese military official looks out an aircraft window during search operations March 13, 2014. Hide Caption 31 of 43 Photos: The search for MH370 Malaysian air force members look for debris near Kuala Lumpur on March 13, 2014. Hide Caption 32 of 43 Photos: The search for MH370 Relatives of missing passengers wait for the latest news at a hotel in Beijing on March 12, 2014. Hide Caption 33 of 43 Photos: The search for MH370 A member of the Vietnamese air force checks a map while searching for the missing plane on March 11, 2014. Hide Caption 34 of 43 Photos: The search for MH370 A Vietnamese air force plane found traces of oil that authorities had suspected to be from the missing Malaysia Airlines plane, the Vietnamese government online newspaper reported on March 8, 2014. However, a sample from the slick showed it was bunker oil, typically used to power large cargo ships, Malaysia's state news agency, Bernama, reported on March 10, 2014. Hide Caption 35 of 43 Photos: The search for MH370 A U.S. Navy Seahawk helicopter lands aboard the USS Pinckney to change crews on March 9, 2014, before returning to search for the missing plane in the Gulf of Thailand. Hide Caption 36 of 43 Photos: The search for MH370 Buddhist monks at Kuala Lumpur International Airport offer a special prayer for the missing passengers on March 9, 2014. Hide Caption 37 of 43 Photos: The search for MH370 Members of a Chinese emergency response team board a rescue vessel at the port of Sanya in China's Hainan province on March 9, 2014. Hide Caption 38 of 43 Photos: The search for MH370 The rescue vessel sets out from Sanya in the South China Sea on March 9, 2014. Hide Caption 39 of 43 Photos: The search for MH370 Malaysian Prime Minister Najib Razak, center, arrives to meet family members of missing passengers at the reception center at Kuala Lumpur International Airport on March 8, 2014. Hide Caption 40 of 43 Photos: The search for MH370 A relative of two missing passengers reacts at their home in Kuala Lumpur on March 8, 2014. Hide Caption 41 of 43 Photos: The search for MH370 Chinese police at the Beijing airport stand beside the arrival board showing delayed Flight 370 in red on March 8, 2014. Hide Caption 42 of 43 Photos: The search for MH370 Malaysia Airlines Group CEO Ahmad Juahari Yahya, front, speaks during a news conference at a hotel in Sepang on March 8, 2014. "We deeply regret that we have lost all contacts" with the jet, he said. Hide Caption 43 of 43

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"But, by the same token, you really have to look through the whole category of people that are on the plane," he said.

Here's what we have so far about some of the people investigators want to know more about:

Pilot: Zaharie Ahmad Shah

Malaysia's Prime Minister has said that somebody deliberately steered the plane off course. That means the pilots have become one obvious area of focus.

On Saturday, Malaysian police searched Zaharie Ahmad Shah's home. The 53-year-old pilot and father of three lives in an upscale, gated community in Shah Alam, outside Malaysia's capital, Kuala Lumpur.

Malaysian police said Sunday they were still investigating a flight simulator seized from that house.

It's somewhat common among aviation enthusiasts to use online flight simulator programs to replicate various situations.

An initial search of the personal computers and e-mails of the pilots found nothing to indicate that a sudden deviation in the Boeing 777's route was preplanned, U.S. officials said on Tuesday, after being briefed by Malaysian authorities.

U.S. officials reviewed cockpit conversations between the flight and air traffic controllers and again said they saw nothing suspicious or anything that would explain why the jetliner deviated from its course.

The pilot's political beliefs are also being questioned. Zaharie is a public supporter of Malaysian opposition leader Anwar Ibrahim.

Just hours before the flight took off, a court ordered Anwar to prison on charges of sodomy, a sentence the opposition leader says is a political vendetta.

Despite the timing of the decision, there is no evidence to tie the plane's disappearance to the pilot or his politics.

"He likely was upset at the verdict that had just been announced several hours before he boarded the aircraft, but to down an airline because of that I think at this point is pure conjecture. Again, I would take any of these accusations with a huge grain of salt," the RAND Corporation's Seth Jones told CNN's "Erin Burnett OutFront."

Peter Chong, a friend of Zaharie's, similarly said it's unfair to imply the pilot had anything to do with what happened to the plane.

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He said he'd been to Zaharie's house and tried out the flight simulator.

"It's a reflection of his love for people," Chong said, "because he wants to share the joy of flying with his friends."

Zaharie joined Malaysia Airlines in 1981 and has more than 18,000 flying hours.

Co-pilot: Fariq Ab Hamid

Fariq Ab Hamid, 27, started at the airline in 2007 and has 2,763 flying hours.

Two vans were loaded with small bags, similar to shopping bags, at the home of the co-pilot, according to a CNN crew that observed activities at the residence.

It was unclear whether the bags were taken from the home, and police made no comment about their activities there.

U.S. intelligence officials are leaning toward the theory that "those in the cockpit" -- the captain and co-pilot -- were responsible for the mysterious disappearance, a U.S. official with direct knowledge of the latest thinking has told CNN.

The official emphasized that no final conclusions have been drawn and that all the internal intelligence discussions are based on preliminary assessments of what is known to date.

Acting Malaysian Transportation Minister Hishammuddin Hussein has told reporters the pilots didn't request to work together.

Passenger: Mohammed Khairul Amri Selamat

The 29-year-old Malaysian civil aviation engineer works for a private jet charter company.

Although police are investigating all passengers and crew, he is likely to be of particular interest because of his aviation knowledge.

"I am confident that he is not involved," his father said Monday. "They're welcome to investigate me and my family."

The bottom line, investigators say, is that whoever flew the plane off course for hours appeared to know what they were doing.

They are looking into the backgrounds of the passengers to see whether any of them were trained pilots.

"There are still a few countries who have yet to respond to our request for a background check," said Khalid Abu Bakar, inspector general of the Royal Malaysian Police Force. "But there are a few ... foreign intelligence agencies who have cleared all the(ir) passengers."

Passengers: Pouri Nourmohammadi and Delavar Seyed Mohammad Reza

In the first few days after the plane went missing, investigators focused intensely on two passengers who boarded the plane using stolen passports.

Authorities have since identified them as Nourmohammadi, 18, and Reza, 29, both Iranians.

The men entered Malaysia on February 28 using valid Iranian passports, according to Interpol.

Malaysian police believe Nourmohammadi was trying to emigrate to Germany using a stolen Austrian passport. His mother contacted police after her son didn't arrive in Frankfurt as expected.

Malaysian investigators say neither of the men has any apparent connection to terrorist organizations.

Stolen passports don't necessarily indicate terrorism. In fact, passengers flew without having their travel documents checked against Interpol's lost-and-stolen passport database more than a billion times in 2013, according to the international police organization.

Terrorist link?

While investigators continue their search, one possibility that has been talked about is that an act of terrorism downed the airliner.

There has been speculation that Uyghur Muslim separatists in China's far western Xinjiang province might have been involved in the plane's disappearance.

The Uyghurs are a predominantly Muslim ethnic group who live in Xinjiang, an area the size of Iran that is rich in natural resources, including oil.

One of the two long corridors where authorities say the plane was last detected stretched over Xinjiang, and unconfirmed reports have suggested the possibility that Uyghurs might be connected to the case.

Chinese authorities have accused separatists from Xinjiang of carrying out a terrorist attack this month in which eight attackers armed with long knives stormed a train station in Kunming, a city in southwestern China, killing 29 people and wounding more than 140.

However, on Tuesday, China said it found no evidence that any of its citizens on board the missing plane were involved in hijacking or terrorism.

Background checks on all passengers from the Chinese mainland on the plane have found nothing to support such suspicions, said Huang Huikang, the Chinese ambassador to Malaysia, according to the state-run Chinese news agency Xinhua.

Authorities have said they are investigating all 239 people who were on board the flight. According to the airline, 153 of the 227 passengers came from mainland China or Hong Kong.

By effectively ruling out suspicions for most of the passengers, Chinese authorities appear to have significantly shortened the list of possible suspects in the investigation.