SAN BERNARDINO, California (REUTERS) - The FBI is investigating this week's massacre in which a married couple killed 14 people in California as an "act of terrorism," an official said on Friday, as sources said that the female shooter apparently pledged allegiance to a leader of the militant group Islamic State in Iraq and Syria (ISIS).

Tashfeen Malik, 27, a native of Pakistan, and her US-born 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 Centre social services agency in San Bernardino, about 100km east of Los Angeles. The attack was the deadliest mass shooting the United States has experienced in three years.

US 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 US officials told Reuters.

They said the finding, if confirmed, could be a "game changer" in the investigation, though another source cautioned there was no indication that ISIS "even knew" who the shooters were.

"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, told reporters.

Investigators have determined that the Malik and Farook engaged in extensive planning before the attack, he said.

Bowdich also said the FBI also was examining crushed cellphones found near the shooting scene and had established that there were "telephonic connections" between the couple and other people of interest in FBI probes.

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 US 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 US 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 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 US government source said.

While the attack may have been inspired by ISIS, there is no evidence that it was directed by the militant group, US 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.

CNN reported on Friday that one US 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.

Bowdich said he was aware of the reported Facebook post and that "we're looking into it."

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 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.

The landlord later asked media to leave.

While investigators left behind receipts for the many weapons and other items seized, the home was filled with domestic items, including two large boxes of diapers and an Arabic tapestry with the 99 names of God that Muslims believe were shared with them through the Koran.

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.

The couple left behind a six-month old daughter. Farook's brother-in-law, Farhan Khan, told NBC News he had begun legal proceedings to adopt the girl and was "very upset and angry" at Farook.

"You left your six-month-old daughter," Khan said. "In this life some people cannot have kids. God gave you a gift of a daughter. And you left that kid behind ... What did you achieve?"

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 US 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.