A woman accused of joining her husband in killing 14 people in California apparently pledged allegiance to a leader of the militant group Islamic State, 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 (100 km) east of Los Angeles. The attack was the deadliest mass shooting the United States has experienced in three years.

U.S. investigators are evaluating evidence that Malik, a Pakistani native who had been living in Saudi Arabia when she married Farook, had pledged allegiance to Islamic State 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.

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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 and the son of Pakistani immigrants, 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 Islamic State, there is no evidence that it was directed by the militant group, U.S. government sources said. Islamic State, 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.

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.

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."

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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.

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