SAN BERNARDINO, California (REUTERS) - A week-old investigation into the shooting rampage that left 14 people dead at a holiday party in Southern California turned to a small lake near the scene of the massacre, where divers searched for evidence on Thursday (Dec 10).

The FBI also revealed that investigators were looking into any connections there may have been between one of the two killers in San Bernardino last week and four men arrested in 2012 in a separate federal terrorism case brought in nearby Riverside, California.

The underwater search stemmed from unspecified leads indicating that the married couple who carried out the shooting spree had been in the vicinity of Seccombe Lake in San Bernardino on the day of the killings, said Mr David Bowdich, assistant director of the FBI field office in Los Angeles.

He said he would not discuss the "specific evidence we're looking for."

CNN reported that investigators were seeking a computer hard drive that belonged to the couple, whom the FBI has said were inspired by Islamic extremists.

Mr Bowdich said investigators had already combed the surrounding park and the search of the lake by FBI and Sheriff's Department divers could take days.

Seccombe Lake Park lies about 4 km north of the Inland Regional Centre, 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 Dec 2 and opened fire with assault rifles.

Farook, 28, a US-born son of Pakistani immigrants, and Malik, 29, a Pakistani native he married last year in Saudi Arabia, were slain hours after their attack in a shootout with police.

The FBI 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 in Iraq and Syria (ISIS), 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 Islamic ideology, as the FBI suspects, it would mark the deadliest such attack on US soil since Sept 11, 2011.

The latest slayings and disclosures about the killers'backgrounds have put law US law enforcement on heightened alert and reverberated into the US presidential campaign, intensifying debates over gun control, immigration and national security.

- POSSIBLE LINKS TO 2012 PLOT EXAMINED -

Speaking to reporters at Seccombe Lake on Thursday, Mr Bowdich disclosed that the FBI was examining whether Farook had any links with the four conspirators in the Riverside case, as CNN and other media outlets have reported.

A jury last year convicted two of those men, including Afghan-born ringleader Sohiel Omar Kabir, of conspiring to provide material support and resources to Al-Qaeda and plotting to attack US military forces in Afghanistan. Their two co-defendants pleaded guilty in the case.

CNN, citing unnamed officials, reported that Farook had been a member of Kabir's social circle. Asked whether the FBI knew of any ties between Farook and the four convicted Riverside conspirators, Mr Bowdich said: "It would be irresponsible not to investigate."

He stressed the four Riverside defendants were never accused of planning attacks within the United States.

FBI Director James Comey told a US Senate committee on Wednesday that the couple were discovered to have been discussing jihad and martyrdom online with each other as far back as 2013, a year before they met in person.

Mr Comey said the precise origins of the couple's radicalisation were as yet unknown but appeared to predate the rise of ISIS, 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.

US government sources told Reuters on Thursday that Malik tried in vain to contact multiple Islamic militant groups in the months before she and Farook staged their attack, but her overtures were ignored.

The organisations that 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 ISIS.

- LAWMAKERS BRIEFED -

While the militant group has since embraced the couple as among its followers, US government sources have said there was no evidence that ISIS even knew of the couple before the San Bernardino killings.

Mr Comey, along with US Homeland Security Secretary Jeh Johnson and John Mulligan, deputy director of the National Counterterrorism Centre, 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."

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 organised 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."

Investigators also have been looking into the couple's ties with Enrique Marquez, a boyhood friend of Farook and Muslim convert who bought the two rifles used in the attack. A federal law enforcement source said Marquez and Farook appear to have contemplated some sort of attack around 2012 but abandoned the idea.

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.