The husband-and-wife attackers who shot and killed 14 people in California on Wednesday had links with the Islamic State (ISIS) and the massacre is now being investigated as an act of terrorism, officials said.

A post on an ISIS-linked blog on Friday also claimed that the attack was carried out by ISIS supporters, the SITE Intelligence Group revealed. The group tracks online communications of extremists.

If eventually confirmed as a terrorist act, the Dec. 2 shooting at the San Bernardino County Health Public Health Department would be the deadliest terrorist attack in the U.S. since Sept. 11, 2001.

At a news conference in Washington on Friday, FBI Director James Comey said investigators are now checking the digital footprints of the killers—Syed Rizwan Farook, 28, and his wife Tashfeen Malik, 27—to determine who else is involved in the plot, the Washington Post reported.

"The investigation so far has developed indications of radicalisation by the killers and of potential inspiration by foreign terrorist organisations," Comey said.

After three days of investigative work, the FBI probers discovered a Facebook post by Malik that linked her to ISIS.

FBI officials said just moments after Farook and Malik mowed down people holding a holiday party at the Inland Regional Center in San Bernardino, Malik went on Facebook and pledged her allegiance to Abu Bakr al-Baghdadi, the ISIS supreme leader.

A Facebook official confirmed the posting. The unnamed official said the company removed the post a day after the attack, adding that Facebook considers praising of ISIS as a violation of its community standards.

Meanwhile, Jihad Watch director Robert Spencer told Newsmax TV that the San Bernardino attacks prove that "the Islamic State has declared jihad against the U.S."

"They have a detailed manual calling for the mass murder for American civilians," Spencer told "The Steve Malzberg Show" in an interview on Thursday. "This attack followed out their plan and also the media's fascination with right-wing extremists.

"You can understand why a so-called ring-wing extremist might want to shoot up an abortion clinic, but a disability centre in a Christmas party? No way."

Police said the husband-and-wife attackers had assembled a huge arsenal of explosives and ammunition in their home, which officials say suggested a degree of planning and raised the possibility of further attacks.

The shooters also tried to cover their tracks by crushing their cell phones aside from other "evidence [indicating that they] attempted to destroy their digital fingerprints," an FBI official in Los Angeles said.

Investigators said the couple were not spotted on the FBI radar and apparently had not taken overt steps to make contact with ISIS operatives.

Records show that Farook was born in Chicago and had a college degree in environmental health and a job as a health inspector. He travelled to Saudi Arabia and Pakistan last year and returned with Malik, whom he had met online, officials said.

Authorities said the two were not on any watch lists.

Two defence attorneys representing Farook's family held a news conference on Friday, describing the husband as a loner and his wife as extremely conservative Muslim who always avoided being in the same room as her male in-laws. Malik's brother-in-law had never seen her face.

Farook and Malik had a six-month-old daughter. The baby is in child protective services, the lawyers said.

During the day of the shooting, police said Farook and Malik were dressed in tactical gear and armed with rifles and handguns and loads of magazines. At about 11 a.m., they entered a conference room where about 80 people were gathered for a staff training session and a holiday party.

They opened fire without warning, hitting more than a third of the people.

The shooters then fled in a rented black Ford Expedition, leaving behind a bag with three pipe bombs designed to be triggered with a remote-control device from their SUV. However, the device malfunctioned, the police said.

Several hours after the shooting, the killer couple's SUV rolled by and police gave chase.

The SUV stopped on San Bernardino Avenue where the two shooters engaged the police in a furious gun battle. Police said the couple fired 76 rifle rounds; police fired 380.

Farook and Malik died at the scene. Two officers were injured, reports said.