WASHINGTON: The militant Islamic State (IS) group ignored contact attempts from Tashfeen Malik in the months before she and her husband killed 14 people at a California holiday party, probably because they feared getting caught in a United States (US) law enforcement sting, US government sources said on Thursday.

The number of organisations that Malik, 29, tried to contact and how she sought to reach them were unclear, but the groups almost certainly included Al Qaeda's Syria-based official affiliate Al-Nusra Front, the government sources said.

One source said investigators have little, if any, evidence that Malik or her husband, Syed Rizwan Farook, 28, had any direct contact with IS, which has seized control of large swaths of Syria and Iraq and claimed responsibility for the Nov 13 Paris attacks that left 130 people dead.

Federal Bureau of Investigation (FBI) Director James Comey has said Malik and Farook declared around the time of their attack that they were acting on behalf of IS ─ which in turn has embraced the couple as among its followers. But US government sources have said there was no evidence that the IS even knew of the couple before the killings.

Militant groups sought out by Malik likely ignored her approaches because they have become extremely wary of responding to outsiders they do not know or who have not been introduced to them, sources said.

Disclosures of Malik's overtures to extremists abroad surfaced as the investigation of the Dec 2 shooting rampage in San Bernardino took a new turn with divers searching a small lake near the scene of the massacre.

On Thursday a team of divers from the FBI and San Bernardino County Sheriff's Department began searching the waters of Seccombe Lake Park, about 2-1/2 miles north of the Inland Regional Center, seeking additional clues in the mass shooting.

The search of the lake, which could last for days, stemmed from an unspecified lead "indicating that the subjects came into this area" on the day of the attack, said David Bowdich, assistant director in charge of the FBI's Los Angeles field office.

"We put a dive team into the lake as a logical part of that lead." Bowdich said he would not discuss the "specific evidence we're looking for", but said it was essentially "anything that had to do with this particular crime". He added: "We may come up with nothing."

CNN reported they sought a computer hard drive that belonged to the couple.

'Not on the radar: US officials search for missed red flags

FBI Director James Comey, along with US Homeland Security Secretary Jeh Johnson and John Mulligan, deputy director of the National Counterterrorism Centre (NCC), briefed members of both houses of Congress on Thursday about the investigation in closed, classified sessions.

The US government appears not to have picked up on extremist messages exchanged during the online courtship two years ago between Farook and his then-fiancée in Pakistan, Tashfeen, federal officials told members of Congress ─ who are concerned red flags may have been missed in the last two years ─ during the closed-door briefings.

American officials say Farook and his wife, Malik, discussed martyrdom and jihad online as early as 2013. But the couple never surfaced on law enforcement's radar and Malik was able to enter the US on a fiancée visa last year despite having reportedly professed radical views online.

"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 cellphones and other electronic equipment.

"Everyone's asking the same questions about how it is that law enforcement didn't know, or intelligence officials didn't know ─ that they could have flown under the radar and nothing gave an indication that they were a threat," said Rep. Jim Langevin, a Rhode island Democrat and member of the House Homeland Security Committee.

Rep. Adam Schiff, the top Democrat on the House Intelligence Committee, said Malik was subjected to an in-person interview during the application process for a visa.

Republican Rep. Will Hurd of Texas, a member of the House Homeland Security Committee, said there's currently no evidence Malik's radicalisation would have been readily apparent when she was evaluated for a fiancée visa. "I don't think there was missed information," he said. "It appears that there was not any evidence that would have been discoverable during an interview for a visa."

He declined to discuss what specifically led investigators to conclude that the couple had radicalised independently as early as 2013.

"It's safe to say that the information about what happened prior to their marriage and to the attacks in San Bernardino was acquired through forensic investigations of these individual lives," Hurd said. "These people weren't on the radar," he added.

Comey, who testified in the Senate on Wednesday, described Farook and Malik as examples of homegrown violent extremists who appear to have radicalised "in place", drawing a distinction between the California attack and the one last month in Paris that officials suspect involved planning and training in Syria.

He said the FBI did not yet know if Farook and Malik's marriage was arranged by a foreign extremist organisation.

'Farook's path towards extremism'

Investigators are trying to determine if Farook's path toward extremism predated 2013 and whether it led to plans to launch an attack in 2012, according to two people familiar with the investigation who were not authorised to discuss the matter publicly and spoke on condition of anonymity.

New revelations show a much deeper connection between Farook and Enrique Marquez, his friend who bought the assault rifles used in the shooting, than previously was disclosed. Marquez has not been charged with a crime.

At least three years ago, Marquez purchased the high-powered weapons that Farook and his wife used in the shooting.

Marquez and Farook "were plotting an actual attack" in 2012, including buying weapons, but became apprehensive and shelved the plan because of law enforcement activity and arrests in the area, said Idaho Sen. Jim Risch, a Republican who sits on the Senate Select Committee on Intelligence.

Marquez, 24, spoke with federal authorities after they raided his mother's Riverside house over the weekend. He and Farook were friends for years and became family last year, with a sister-in-law in common.

The two men were listed as witnesses on the marriage license when Farook's brother, Raheel, wed a Russian woman in 2011.

Three years later, Raheel Farook and his wife, Tatiana, were witnesses to Marquez's marriage to her sister, Mariya Chernykh, according to Riverside County records.

The ceremony took place at the Islamic Society of Corona-Norco, according to the marriage license, though the mosque's facility manager denied it occurred there.

Malik's father, reached in Jeddah, Saudi Arabia, condemned his daughter's actions and said he is "very, very sad. ... I am in such pain that I cannot even describe it."

The father, Gulzar Ahmed Malik, has been a resident in the kingdom since the early 1980s, the Saudi Interior Ministry said. His daughter was from Pakistan but travelled to Saudi Arabia.

California shooting

Syed Rizwan Farook and Tashfeen Malik were killed in a shootout with police hours after they opened fire with assault rifles at a holiday gathering of Farook's co-workers at the Inland Regional Center social services agency in San Bernardino.

Fourteen people were killed, and the number of wounded was raised to 22 on Thursday from 21 after a woman injured in the assault came forward, officials said.

The FBI said it is treating the shooting attack as an act of terrorism. If the massacre proves to have been the work of killers inspired by militants, it would mark the deadliest such attack on US soil since Sept 11, 2011.

The attack has heightened security concerns in the US and become an issue in the US presidential campaign.