A still image from a video footage courtesy of Nbcla.com shows first responders responding to a shooting at the California Department of Developmental Services Inland Regional Center, California December 2, 2015.

The Islamic State of Iraq and Syria (ISIS) group said in an online radio broadcast on Saturday that two followers of the militant group had carried out Wednesday’s attack on a social services agency party in California in which 14 people were killed.

“Two followers of Islamic State (ISIS) attacked several days ago a center in San Bernadino in California.” the group’s daily broadcast al-Bayan said.

A news agency which supports ISIS said on Friday the attackers were followers of the group. U.S. government sources have said there is no evidence the attack was directed by the militant group, or that the organization even knew who the attackers were.

FBI investigation

The FBI began investigating the fatal shooting of 14 people in California by a married couple as an “act of terrorism,” officials said on Friday, noting the wife was believed to have pledged allegiance to a leader of the militant group ISIS.

While the FBI said it lacked evidence the couple belonged to a larger organization of extremists, the Los Angeles Times cited a federal law enforcement source in reporting that the husband, Syed Rizwan Farook, 28, had contact with at least two militant groups overseas, including the al Qaeda-affiliated al-Nusra Front in Syria.

Both the U.S.-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 social services agency in San Bernardino, about 60 miles (100 km) east of Los Angeles.

If the mass shooting proves to have been the work of people inspired by militants, as investigators now suspect, it would mark the deadliest such attack in the United States since Sept. 11, 2001.

Federal Bureau of Investigation officials said mounting signs of advanced preparations, the large cache of armaments amassed by the couple and evidence that they “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,” David Bowdich, assistant director of the Federal Bureau of Investigation’s Los Angeles office, said at a news conference.

Bowdich 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. And Bowdich said they may have been planning an additional attack.

Pledge of allegiance

One startling disclosure came from social media network Facebook, which confirmed that comments praising ISIS were posted around the time of the mass shooting to a Facebook 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.

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 of “acts of terror.” He declined to elaborate on the material.

But CNN and other news media outlets reported that Malik’s Facebook posts included a pledge of allegiance to ISIS leader Abu Bakr al-Baghdadi.

Asked about a reported Facebook post by Malik on the day of the attack pledging loyalty to ISIS, 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 ISIS, there was no evidence the attack was directed by the militant group, or that the organization even knew who they were, U.S. government sources said. ISIS, which has seized large swaths of Syria and Iraq, claimed responsibility for the Nov. 13 attacks in Paris in which gunmen and suicide bombers killed 130 people.

Speaking to reporters separately in Washington, FBI Director James Comey said the investigation pointed to “radicalization of the killers and of potential inspiration by foreign terrorist organizations.”

But no evidence has been uncovered yet suggesting the killers were “part of an organized larger group, or form part of a cell,” Comey said. “There is no indication that they are part of a network.”

Bowdich said neither Farook nor Malik had been under investigation by the FBI or other law enforcement agency 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.

Citing an unnamed federal law enforcement official, the Los Angeles Times reported late on Friday that Farook had “some kind” of contact with people from the al-Nusra Front and the radical Shabab group in Somalia. But the nature of that contact and with whom was unclear, the Times said.

This undated combination of photos provided by the FBI, left, and the California Department of Motor Vehicles shows Tashfeen Malik, left, and Syed Farook.

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