The FBI is investigating the fatal shooting of 14 people in California by a married couple as an act of terror and believes the wife pledged allegiance to an Islamic State leader.
The bureau said on Friday it lacked evidence the pair belonged to a larger organisation of extremists.
However, the Los Angeles Times cited a federal law enforcement source in reporting the husband, Syed Rizwan Farook, 28, had contact with at least two militant groups overseas, including the al Qaeda-affiliated Nusra Front in Syria.
Both the US-born husband and his spouse, Tashfeen Malik, 29, a native of Pakistan who lived in Saudi Arabia for more than 20 years, died in a shootout with police hours after Wednesday's attack on a holiday party at the Inland Regional Center in San Bernardino.
If the mass shooting proves the work of people inspired by Islamist militants, as investigators now suspect, it would mark the deadliest such attack in the United States since September 11, 2001.
FBI officials said mounting signs of advanced preparations, the large cache of armaments amassed and evidence the couple "attempted to destroy their digital fingerprints" helped tip the balance of the investigation.
"Based on the information and the facts as we know them, we are now investigating these horrific acts as an act of terrorism," assistant FBI Los Angeles director David Bowdich said.
Bowdich said the FBI hoped examination of data retrieved from two smashed mobile phones and other electronic devices seized in the investigation would lead to a motive.
The couple had two assault-style rifles, two semi-automatic handguns, 6100 rounds of ammunition and 12 pipe bombs in their home or with them when they were killed, officials said. And Bowdich said they may have been planning an additional attack.
One startling disclosure came from Facebook, which confirmed comments praising Islamic State were posted around the time of the mass shooting to an account established under an alias by Malik.
However, it was uncertain whether the comments were posted by Malik herself or someone with access to her page.
But CNN and other news media outlets reported that Malik's Facebook posts included a pledge of allegiance to Islamic State leader Abu Bakr al-Baghdadi.
Asked about a reported Facebook post by Malik on the day of the attack pledging loyalty to Islamic State, Bowdich said, "I know it was in a general timeline where that post was made, and yes, there was a pledge of allegiance."
While Malik and her husband may have been inspired by Islamic State, there was no evidence the attack was directed by the militant group, or that the organisation even knew who they were, US government sources said.
Speaking separately in Washington, FBI Director James Comey said the investigation pointed to "radicalisation of the killers and of potential inspiration by foreign terrorist organisations".
Bowdich said neither Farook nor Malik had been under investigation by the FBI or other law enforcement agencies prior to Wednesday.
And none of the contacts federal agents have since discovered between the couple and the subjects of other FBI inquiries "were of such a significance that it raised these killers up onto our radar screen," Comey said.
Farook family attorneys deny there is any evidence either the husband or wife harboured extremist views.
"She was like a typical housewife," lawyer David Chesley said, describing Malik as "caring, soft-spoken" and a devout Muslim who prayed five times a day, chose not to drive and "kept pretty well isolated".
They said Farook, too, largely kept to himself, had few friends and said co-workers sometimes made fun of his beard.
Farook, born in Illinois to Pakistani parents, earned more than $US50,000 ($A68,000) a year as an inspector for the San Bernardino County Department of Environment Health, the agency whose holiday party he and Malik are accused of attacking.
Investigators are looking into a report he had an argument with a co-worker who denounced the "inherent dangers of Islam".
