Syed Rizwan Farook

WASHINGTON — A woman who joined her husband in killing 14 people in California apparently pledged allegiance to a leader of ISIS, two U.S. government sources said on Friday, as intelligence officials in her native Pakistan pressed the investigation overseas.

Tashfeen Malik, 27, and her husband, Syed Rizwan Farook, 28, were killed in a shootout with police hours after the Wednesday massacre during a holiday party at the Inland Regional Center social services agency in San Bernardino, about 60 miles east of Los Angeles. 

“As of today, based on the information and the facts as we know them, we are now investigating these horrific acts as an act of terrorism,” Dave Bowdich of the FBI said at a Friday morning news conference.

The attack was the deadliest mass shooting the United States has experienced in three years.

Investigators are evaluating evidence that Malik, a Pakistani native who had been living in Saudi Arabia when she married Farook, had pledged allegiance to ISIS leader Abu Bakr al-Baghdadi, two U.S. officials told Reuters. They said the finding, if confirmed, could be a “game changer” in the investigation.

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. A U.S. government source said because of the amount of weaponry, investigators are trying to determine if they intended to carry out a more elaborate attack.

Farook, a U.S. citizen 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 allegedly attacked 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 U.S. government source said.

While the attack may have been inspired by ISIS, there is no evidence that it was directed by the militant group, U.S. government sources said. ISIS, which has taken control of large parts of Syria and Iraq, took claim for the Nov. 13 attacks in Paris in which gunmen and suicide bombers killed 130.

“The FBI is leading this investigation because of the possibility that this is a terrorist attack,” White House spokesman Josh Earnest told reporters.

CNN reported on Friday that one U.S. official said Malik had made the pledge to al-Baghdadi in a posting on Facebook on Wednesday, the day of the attack, under an account that used a different name.

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 influx of media drew a barrage of criticism on social media, although FBI spokeswoman Lourdes Arocho said the bureau had concluded its search of the home.

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 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.

Before going on their rampage on Wednesday, Malik and Farook had destroyed computer hard drives and other electronics, a U.S. government source said.