San Bernardino, Calif./Washington: The FBI is investigating this week’s massacre of 14 people by a married couple in California as an ‘act of terrorism,’ officials said on Friday, noting that the female shooter had pledged allegiance to a leader of the militant group Islamic State.

Tashfeen Malik, 27, a native of Pakistan who lived in Saudi Arabia for more than 20 years, and her U.S.-born husband, Syed Rizwan Farook, 28, were killed in a shootout with police hours after the Wednesday attack during 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 investigation does prove that the attack was the work of people inspired by Islamist militants, it would be the deadliest such attack in the United States since 11 September, 2001.

Malik had pledged allegiance to Islamic State leader Abu Bakr al-Baghdadi in an online post, David Bowdich, assistant director of the Federal Bureau of Investigation’s Los Angeles office, said at a news conference.

‘Based on the information and the facts as we know them, we are now investigating these horrific acts as an act of terrorism,’ Bowdich said.

Asked about a reported Facebook post by Malik on the day of the attack pledging loyalty to Islamic State, Bowdich said, ‘Yes, there was a pledge of allegiance.’

But a US government official said there was no evidence that Islamic State even knew who the shooters were.

Islamic State, which has seized large parts of Syria and Iraq, took claim for the 13 Novemebr attacks in Paris in which gunmen and suicide bombers killed 130.

Farook family attorneys said there was no evidence that either suspect had been associated with any terrorist group.

They said Farook was an isolated individual with few friends, and there was no evidence that either suspect had extremist views.

The family was aware that Farook owned two handguns and said coworkers had made fun of his beard, the attorneys said.

The family described Malik as a ‘caring, soft-spoken’ housewife.

No signs of a cell

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.

‘The investigation so far has delivered indications of radicalization by the killers and of potential inspiration by foreign terrorist organizations,’ Federal Bureau of Investigation Director James Comey told reporters in Washington. ‘We have no indication that these killers are part of an organized larger group or form part of a cell. There is no indication that they are part of a network.’

Bowdich also said the FBI was examining crushed cellphones found near the shooting scene.

The couple may have been planning an additional attack, he added.

Farook, born in Illinois to Pakistani immigrant parents, worked as an inspector for the San Bernardino County Department of Environment Health, the agency whose holiday party he and Malik are accused of attacking on Wednesday.

Investigators are looking into a report that Farook had an argument with a co-worker who denounced the ‘inherent dangers of Islam’ prior to the shooting, a US government source said.

The couple’s landlord in the town of Redlands opened their townhouse to media on Friday, leading to a flurry of reporters and camera crews surveying the scene. The landlord later asked media to leave.

The FBI’s search of the home turned up no evidence to suggest they had been working with any foreign militant group, a US government source said.

Probe extends to Pakistan

Pakistani intelligence officials have contacted Malik’s family in her homeland as part of the investigation, a family member said.

‘I only found out about this tragedy today when some intelligence officials contacted me to ask me about my links with Tashfeen,’ Malik’s uncle, Javed Rabbani, said in an interview. ‘I had heard in the news that this tragedy had taken place but I could never even imagine that it would be someone from my family. Of course, we are in shock.’

He said his brother, Malik’s father, had become considerably more conservative since moving with his family to Saudi Arabia a quarter century ago.

Tashfeen Malik had not come to the attention of authorities while living in Saudi Arabia, according to a source close to the Saudi government. She had moved back to Pakistan five or six years ago to study pharmacy, Pakistani officials said.

Christian Nwadike, who worked with Farook for five years, told CBS that his co-worker had been different since he returned from Saudi Arabia.

‘I think he married a terrorist,’ Nwadike said.

Twenty-one people were wounded in the attack, the worst gun violence in the nation since the December 2012 shooting at Sandy Hook Elementary School in Newtown, Connecticut.

Farook had not been under surveillance by the FBI or any other law enforcement agency, Bowdich said, adding that there was no evidence that additional threats lingered following the shooting.