There is no indication that the shooters who killed 14 people in California are part of a larger terrorist group, according to the White House.
The statement comes after the so-called Islamic State group claimed the pair were "soldiers" of its caliphate.
A team of top officials including FBI Director James Comey, Attorney General Loretta Lynch and Secretary of Homeland Security Jeh Johnson told US President Barack Obama "that they had as of yet uncovered no indication the killers were part of an organized group or formed part of a broader terrorist cell," but pointed to evidence that they had been "radicalized to violence," the White House said in a statement.
Earlier, IS said the married couple were followers of the militant group based in Syria and Iraq.
The group's declaration, in an online radio broadcast, came three days after US-born Syed Rizwan Farook, 28, and his wife Tashfeen Malik, 29, a native of Pakistan, carried out the attack on a holiday party for civil servants in San Bernardino, about 100km east of Los Angeles.
The two died hours later in a shootout with police.
The party the couple attacked was for workers in the same local government agency that employed Farook.
If Wednesday's mass shooting proves to have been the work of people inspired by Islamist militants, as investigators now suspect, it would mark the deadliest such attack in the United States since 11 September 2001.
Two ethnic Chechen brothers inspired by al-Qaeda killed three people and injured 264 in a bombing attack on the Boston Marathon in April 2013.
"Two followers of Islamic State attacked several days ago a centre in San Bernadino in California," the group's daily online radio broadcast al-Bayan said this morning.
An English-language version of the broadcast was later released calling the attackers "soldiers" of Islamic State, rather than "followers" as in the original Arabic version.
It was unclear if the English version was claiming them as members, or why there was an inconsistency.
The broadcast came a day after Facebook confirmed that comments praising Islamic State were posted around the time of the mass shooting to an account on the social media website stablished by Malik under an alias.
However, it was uncertain whether the comments were posted by Malik herself or someone with access to her page.
"There was a pledge of allegiance," David Bowdich, assistant director of the Federal Bureau of Investigation's Los Angeles office, told a news conference about a reported loyalty pledge posted on Facebook by Malik on the day of the attack.
A Facebook Inc spokesman said the profile in question was removed by the company on Thursday for violating its community standards barring promotion or praise for "acts of terror".
He declined to elaborate on the material.
CNN and other news media outlets reported the Facebook posts on Malik's page included a pledge of allegiance to Islamic State leader Abu Bakr al-Baghdadi.
"We are now investigating these horrific acts as an act of terrorism," Mr Bowdich told reporters.
He said the FBI hoped examination of data retrieved from two smashed cellphones and other electronic devices seized in the investigation would lead to a motive for the attack.
The couple had two assault-style rifles, two semi-automatic handguns, 6,100 rounds of ammunition and 12 pipe bombs in their home or with them when they were killed, officials said.
Mr Bowdich said they may have been planning an additional attack.
The attack prompted the New York Times to run its first front-page editorial since 1920, saying it was "a moral outrage and a national disgrace" that the sort of firearms used in the attack were so readily available.
