An FBI dive team searches for any evidence that may have been left by the couple behind the San Bernardino massacre at Seccombe Lake Park in San Bernardino, California, on December 10, 2015 PHOTO: AFP

US government sources told Reuters on Thursday that Malik tried in vain to contact multiple militant groups in the months before she and Farook staged their attack, but her overtures were ignored.The organisations Malik sought out likely shied away out of extreme caution in communicating with individuals unknown to them and a fear of being caught up in a law-enforcement "sting" operation, sources said.The number of organisations that Malik attempted to approach and how she sought to reach them were unclear, though the groups almost certainly included al Qaeda’s Syria-based official affiliate, the Nusrah Front, the government sources said.One source said investigators have little, if any, evidence that Malik or her husband had any direct contact with Islamic State.While the militant group has since embraced the couple as among its followers, US government sources have said there was no evidence Islamic State even knew of the couple before the San Bernardino killings.Comey, along with US Homeland Security Secretary Jeh Johnson and John Mulligan, deputy director of the National Counterterrorism Center, briefed members of both houses of Congress on Thursday about the investigation in closed, classified sessions."The current impression is that these two people were acting alone," US Senator Angus King of Maine told CNN after the briefing. But he added that he was troubled by the fact that the couple had tried to cover their tracks by destroying their cell phones and other electronic equipment.Representative Bob Goodlatte of Virginia, the Republican chairman of the House Judiciary Committee, told reporters afterward that there were people in the community who saw suspicious activity at the shooters' house but decided not to tell authorities "for a variety of reasons."A week-old investigation into the shooting rampage that left 14 people dead at a holiday party in Southern California turned on Thursday to a small lake near the scene of the massacre, where divers searched the water's depths for new evidence in the case.Investigators were drawn to Seccombe Lake in San Bernardino, California, by unspecified leads indicating the married couple who carried out the shooting had been in the vicinity the day of the killings, said David Bowdich, assistant director of the FBI field office in Los Angeles.Bowdich said he would not discuss the “specific evidence we’re looking for.”CNN reported investigators were seeking a computer hard drive that belonged to the couple, whom the FBI has said were inspired by extremists. Bowdich said investigators already have combed the surrounding park, and that the search of the lake could take days.Seccombe Lake Park lies about 2.5 miles (4 km) north of the Inland Regional Center, the social services agency where 14 people were killed and 22 others injured when Syed Rizwan Farook and his wife, Tashfeen Malik, stormed a holiday gathering of his co-workers there on December 2 and opened fire with assault rifles.Farook, 28, a US-born son of Pakistani immigrants, and Malik, 29, a Pakistani native he married in Saudi Arabia last year, were slain hours later in a shootout with police.The Federal Bureau of Investigation said it is treating the mass shooting as an act of terrorism, citing the couple's declaration that they were acting on behalf of the militant group Islamic State, as well as a large cache of weapons, ammunition and bomb-making materials seized in the investigation.The couple's motives remain unclear. But if the crime proves to have been the work of killers driven by militant ideology, as the FBI suspects, it would mark the deadliest such attack on US soil since September 11, 2011.The attack and disclosures about the killers' backgrounds have put US law enforcement on heightened alert and reverberated on the US presidential campaign trail, intensifying debates over gun control, immigration and national security.FBI Director James Comey told a US Senate committee on Wednesday that the couple were discovered to have been discussing martyrdom online with each other as far back as 2013, a year before they met in person.Comey said the precise origins of the couple's radicalization were as yet unknown but appeared to predate the rise of Islamic State, the militant group that has seized vast swaths of Iraq and Syria and last month claimed responsibility for the assaults on Paris that left 130 dead.New York Representative Peter King, a senior Republican member of the House Intelligence Committee, emerged from the session calling for "more surveillance in the Muslim community here in the United States."He drew comparisons to heightened scrutiny law enforcement placed on Italian-American and Irish-American communities during past investigations of organized crime."You look where the terror is going to come from, and right now, it is going to come from the Muslim community," he said. "It's a small percentage, but to me, the only way you find out about it in advance is having sources and informers on the ground, having constant surveillance."Farook and Malik had contacts with people in Orange County, California, who the FBI had been watching for possible ties to terrorism, but nothing arose during that investigation to draw attention to either shooter, a US government source said. The source added there is no evidence the shooters had plotted with anyone who had come under FBI scrutiny.Investigators also have been looking into the relationship between Farook and Enrique Marquez, a boyhood friend and Muslim convert who purchased the two rifles used in the attack. Another federal law enforcement source said Marquez and Farook had plotted some sort of attack around 2012 but abandoned it.Marquez, who is related to Farook's family by marriage - his wife and the wife of Farook's older brother are sisters - has not been charged with any crime.