The suicide bomber who killed seven CIA employees at a remote outpost in southeastern Afghanistan had been invited onto the base and had not been searched, two former U.S. officials told The Associated Press on Thursday 
 
 A former senior intelligence official says the man was being courted as an informant and that it was the first time he had been brought inside the camp. An experienced CIA debriefer came from Kabul for the meeting, suggesting that the purpose was to gain intelligence, the official said. 
 
 The former intelligence official and another former official with knowledge of the attack spoke on condition of anonymity because they were not authorized to speak publicly. 
 
 The CIA would not confirm the details, and said it was still gathering evidence on the incident. 
 
 "It's far too early to draw conclusions about something that happened just yesterday," said spokesman George Little. 
 
 A separate U.S. official suggested the bomber may have set off the explosives as he was about to be searched. 
 
 The bombing on Wednesday dealt a blow to the tight-knit spy agency. Among those killed was the chief of the CIA post, whom former officials identified as a mother of three. Six more agency personnel were wounded in what was considered the most lethal attack for the CIA since the war in Afghanistan began in 2001 and possibly even since the 1983 embassy bombing in Beirut. 
 
 It also was the single deadliest attack for Americans in Afghanistan since eight soldiers were killed in an insurgent attack on a base in the east on Oct. 3. 
 
 President Barack Obama and CIA Director Leon Panetta were joined by several leading lawmakers on Thursday in praising agency employees for their work. 
 
 "Those who fell yesterday were far from home and close to the enemy, doing the hard work that must be done to protect our country from terrorism," Panetta said in a statement confirming the deaths. "We owe them our deepest gratitude, and we pledge to them and their families that we will never cease fighting for the cause to which they dedicated their lives _ a safer America." 
 
 In a letter to CIA employees, Obama said their fallen colleagues came from a "long line of patriots" who had helped to keep the nation safe despite grave risks. 
 
 Obama acknowledged that the spy agency has been tested "as never before" since the attacks of Sept. 11, 2001. 
 
 The CIA did not release information about the victims, citing the sensitivity of their mission and other ongoing operations. 
 
 According to one former agency employee, the death toll represents a significant portion of the CIA's clandestine force in the region, but that many of the agency's employees have experience in Afghanistan. 
 
 "The bench is deeper in Afghanistan than it is anywhere in the world," the former employee said. 
 
 The bigger question for CIA operations will probably be whether the agency moves to tighten safety rules for its employees, the former employee said. 
 
 The incident occurred at a former military base on the edge of Khost city, the capital of Khost province which borders Pakistan and is a Taliban stronghold. 
 
 The Taliban claimed responsibility. 
 
 Taliban spokesman Zabiullah Mujahid said in a statement that an Afghan National Army officer wearing a suicide vest entered the base and blew himself up inside the gym. A U.S. official briefed on the blast also said it took place in the gym. 
 
 Forward Operating Base Chapman used to be a military facility base but was later turned into a CIA base, according to a U.S. official, who spoke on condition of anonymity because he was not authorized to speak publicly. 
 
 Some military men and women work there on a Provincial Reconstruction Team, one of several civilian-military units that secure and develop areas of Afghanistan. A NATO spokesman said other personnel operate from Chapman as well, but he said he could not elaborate. 
 
 Only four known CIA operatives have been killed in Afghanistan since the 9/11 terrorist attacks in the U.S. 
 
 CIA officer Micheal "Mike" Spann was killed in a prison uprising in November 2001. An agency officer died in a training exercise in 2003, and two contractors operating out of a CIA base in Shkin district of Paktika province were killed the same year. 
 
 ___ 
 
 Adam Goldman reported from New York. Associated Press writers Matthew Lee and Anne Flaherty contributed to this report. ||||| WASHINGTON—Wednesday's attack on a U.S. compound in Afghanistan devastated what has been a hub of counterterrorism and intelligence operations for the spy agency. 
 
 Seven Central Intelligence Agency officers and contractors were killed and six more wounded in the suicide bomb attack at Forward Operating Base Chapman, CIA Director Leon Panetta said Thursday, the second-largest single-day loss for the spy agency in its history. 
 
 Among the casualties was the agency's base chief, former intelligence officials said. There had been only four publicly acknowledged CIA fatalities in Afghanistan prior to this attack. 
 
 The Taliban claimed responsibility Thursday for the bombing, which was carried out by suicide bomber wearing an Afghan National Army uniform. Some senior officials think the bomber may have been given access to the base because he was believed to be an informant, said two former intelligence officials. 
 
 Several former intelligence officials described the attack in Afghanistan as "devastating" to the agency. A number of the officers killed had been counterterrorism operatives since before the 9/11 attacks. The base played a critical role in the CIA's significant operations in the country, including helping with drone attacks and informant networks in Pakistan. 
 
 The loss of seven officers is significant for a relatively small agency whose workforce is estimated to be 10,000 or more, but it's all the more damaging because those lost represented so much collective experience. 
 
 Enlarge Image Close Reuters Canadian soldiers patrol in the southern city of Kandahar on December 31. 
 
 They were "experienced frontline officers and their knowledge and expertise will be sorely missed" and not easily regenerated, said Henry A. Crumpton, who led the CIA campaign in Afghanistan in 2001 and 2002. 
 
 The number of casualties in Wednesday's attack was second to those sustained in the Beirut embassy bombing in 1983, which killed eight CIA officers. The Beirut bombing hit the agency's Middle East group hard, and was one of the key events that drove the creation of the CIA's Counterterrorist Center a few years later. 
 
 "It will mark this generation the same way Beirut marked mine," said Ron Marks, a 16-year CIA veteran, who left the agency in 1999. With CIA officers deployed to the far reaches of Afghanistan and Pakistan for extended periods, he said, the agency has been lucky to have avoided such attacks for as long as it did. 
 
 It could also sow mistrust between CIA officers and the Afghan operatives with whom they work closely, another former agency officer said. ""This is a huge blow to the agency. It's a close-knit group," the former officer said. "They're not going to know who to trust now." 
 
 The base was located in Khost province, a hotbed of militant activity and a stronghold of the Haqqani Network, one of the most hardened and dangerous militant groups, which fought for a decade to wrest the area back from the Russians 
 
 The CIA's Khost base was established in the months after the 9/11 attacks as the U.S. launched its CIA-led offensive against al Qaeda and the Taliban in Afghanistan. It began as a makeshift center for CIA-Afghan operations. By mid-2002, it had grown into a major counterterrorism hub for joint operations with CIA, military Special Operations forces and Afghan allies. 
 
 The Taliban says it's responsible for two deadly bombings Wednesday, one inside a CIA base and another that killed Canadian troops and a journalist embedded with them. Video courtesy of Reuters. 
 
 Its primary role has been to run informant networks in Afghanistan and over the border, said one former agency official. "That was one of the bases where they were paying people and running people and sending them into Pakistan," he said. 
 
 The CIA's activities on the base were an open secret locally, he added, "al Qaeda knows it and the townspeople know it and the Taliban know it." 
 
 The attack in Afghanistan came during an already difficult week for the CIA, which has taken a beating in Washington with President Barack Obama issuing a blunt critique of intelligence failures in advance of the botched Christmas Day terrorist attack. 
 
 The brazen assault may prompt a re-examination how CIA deploys operatives into dangerous tribal regions in Afghanistan and Pakistan, and how local employees and other local operatives working with the CIA are vetted, former intelligence officials said. 
 
 "Those who fell yesterday were far from home and close to the enemy, doing the hard work that must be done to protect our country from terrorism," Mr. Panetta said in a message to agency employees. "We owe them our deepest gratitude, and we pledge to them and their families that we will never cease fighting for the cause to which they dedicated their lives—a safer America." 
 
 Mr. Panetta credited military doctors and nurses with saving the lives of the wounded. He said the agency would not release the identities of the officers killed at this time. 
 
 "Yesterday's tragedy reminds us that the men and women of the CIA put their lives at risk every day to protect this nation," Mr. Panetta said. 
 
 Eight Americans are killed in an explosion at a U.S. compound in Afghanistan, officials say. Reporting from Kabul, WSJ's Anand Gopal joins the Hub to discuss. Plus, a look at what investors can expect from the market in 2010 as well as a growing trend in the pet community -- toilet-training cats. 
 
 President Barack Obama also wrote to CIA employees Thursday to praise the service of officers who were killed. 
 
 "These brave Americans were part of a long line of patriots who have made great sacrifices for their fellow citizens and for our way of life," he wrote. Since the Sept. 2001 terrorist attacks, he said, "because of your service, plots have been disrupted, American lives have been saved, and our Allies and partners have been more secure." 
 
 The CIA began mobilizing against the perpetrators of the attack. "The CIA is already working hard to find those who supported the Khost attack," a U.S. intelligence official, adding "this attack will be avenged through successful, aggressive counterterrorism operations." 
 
 CIA spokesman George Little said that the attack serves as a reminder of the dangerous nature of the CIA's work. "There's still a lot to be learned about what happened," he said. "The key lesson is that counterterrorism work is dangerous. Our fallen and wounded colleagues were on the front lines, conducting essential operations to protect our country." 
 
 On Capitol Hill, the leaders of the House and Senate intelligence committees issued statements of condolence. 
 
 Wednesday's casualties will be added to a wall in the CIA's lobby, which currently features 90 stars representing agency employees killed in the line of duty. The most recent one was added in June to memorialize an officer killed last year, but the officer's name and duties weren't made public. 
 
 According to a military official who works on Afghan issues, Chapman has grown substantially in recent months and is a base for both military and intelligence operations. Because of its size, the officer said, the suicide bomber likely penetrated multiple layers of security before detonating the explosives. 
 
 Much about the attack at Forward Operating Base Chapman remained uncertain. Officials variously said the blast had occurred as the bomber exited a car, or after the bomber had reached the base's gym or its cafeteria. 
 
 Enlarge Image Close Associated Press A demonstrator in Kabul on Wednesday rallied against a recent U.S.-led attack in Afghanistan that protestors and the Afghan government say killed children. NATO disputed the claim. 
 
 Forward operating bases typically house hundreds of soldiers, and Afghan forces and private contractors also often live on such bases. But CIA outposts on these bases are usually small—no more than 15 or maybe 20 people, so 13 casualties is likely a majority of the CIA base personnel, said one former agency official. 
 
 The attack appeared to be the worst against foreigners since October, when 10 Americans -- seven troops and three civilians -- were killed in a helicopter crash following a firefight with insurgents. 
 
 It would also mark the first time a suicide bomber managed to strike inside a U.S. facility in the country, a sign of the insurgents' growing sophistication. Insurgents have been staging increasingly complicated assaults in recent months, including one where a militant infiltrated the country's police force and killed five British soldiers. 
 
 In a press release Thursday, the Taliban claimed responsibility for the Khost attack. "The martyr attack was carried out by an Afghan military, Samiullah, when the CIA agents got together in a U.S. club-cum-training center for a meeting in order to collect information on the Mujahiodeen," the release said, according to a translation by the Middle East Media Research Institute. 
 
 "According to one of the club's guards, the CIA employees, dressed in Afghan uniform were in the club discussing as an army officer entered the club and set off his explosive-packed vest, killing and injuring all inside the club." 
 
 —Anand Gopal contributed to this article. 
 
 Write to Siobhan Gorman at siobhan.gorman@wsj.com
– Among the seven CIA employees and contractors killed in yesterday's suicide bombing in Afghanistan was the agency's chief officer at the post. One former intelligence official called the attack "devastating" to the CIA's operations in the country. "There was some tremendous talent lost," the official tells the Wall Street Journal. The blast marks grim milestones: It was the deadliest strike against Americans in Afghanistan since October, and it's the first time a suicide bomber struck inside a US facility there. How it happened remains unclear. The AP, quoting anonymous former officials, says the bomber may have been invited to the Chapman military base as a potential informant. He hadn't been searched when he set off the explosives. "Yesterday's tragedy reminds us that the men and women of the CIA put their lives at risk every day to protect this nation," said CIA chief Leon Panetta. Six others were wounded in the blast.