SAN BERNARDINO, Calif.  — The FBI said Friday that it is officially investigating the mass shooting in California as an act of terrorism, while a U.S. law enforcement official revealed that the woman who helped her husband carry out the attack had pledged allegiance to the Islamic State group and its leader on Facebook under an alias.

David Bowdich, assistant director of the FBI’s Los Angeles office, would not give further details about why the bureau made the determination, saying at a news conference that “there’s a number of pieces of evidence that has pushed us off the cliff.”

Syed Farook and his wife, Tashfeen Malik, killed 14 people at the holiday party for his co-workers. The Muslim couple died hours later in a fierce gunbattle with police.

A Facebook official says Tashfeen Malik praised the leader of the Islamic State group in a post at 11 a.m. Wednesday, when the couple were believed to have stormed a San Bernardino social service center and opened fire.

The Facebook official, who spoke on condition of anonymity because the person was not allowed under corporate policy to be quoted by name, said the company discovered the account Thursday. It removed the profile from public view and reported its contents to law enforcement.

Malik, 27, was a Pakistani who grew up in Saudi Arabia and came to the U.S. in 2014 on a fiancée visa. Farook, a 28-year-old restaurant health inspector for the county, was born in Chicago to Pakistani parents and raised in Southern California.

Another U.S. official said Malik expressed “admiration” for the extremist group’s leader on Facebook under the alias account. But the official said there was no sign that anyone affiliated with the Islamic State communicated back with her, and there was no evidence of any operational instructions being conveyed to her.

The two U.S. officials were not authorized to discuss the case publicly and spoke on condition of anonymity.

The FBI has been investigating the shooting at a social service center as a potential act of terrorism but had reached no firm conclusions as of Thursday, with authorities cautioning repeatedly that the violence could have stemmed from a workplace grudge or a combination of motives.

Separately, a U.S. intelligence official said on Thursday that Farook had been in contact with known Islamic extremists on social media.

Law enforcement officials have long warned that Americans acting in sympathy with Islamic extremists — though not on direct orders — could launch an attack inside the U.S. Using slick propaganda, the Islamic State in particular has urged sympathizers worldwide to commit violence in their countries.

Others have done so. In May, just before he attacked a gathering in Texas of people drawing cartoons of the Prophet Muhammad, a Phoenix man tweeted his hope that Allah would view him as a holy warrior.

Two weeks ago, with Americans on edge over the Islamic State attacks in Paris that left 130 people dead, FBI Director James Comey said that U.S. authorities had no specific or credible intelligence pointing to an attack on American soil.

Seventy-one people have been charged in the U.S. since March 2014 in connection with supporting ISIS, including 56 this year, according to a recent report from the George Washington University Program on Extremism. Though most are men, “women are taking an increasingly prominent role in the jihadist world,” the report said.

It was not immediately clear whether Malik exhibited any support for radical Islamists before she arrived in the U.S. — or, like scores of others arrested by the FBI, became radicalized through online or in-person associations after arriving.

To receive her visa, Malik was subjected to a vetting process the U.S. government describes as vigorous. It includes in-person interviews, fingerprints, checks against terrorist watch lists and reviews of her family members, travel history and places where she lived and worked.

Foreigners applying from countries that are home to Islamic extremists — such as Pakistan — undergo additional scrutiny before the State Department and Homeland Security approve their applications.

Pakistani intelligence officials said Malik moved as a child with her family to Saudi Arabia 25 years ago.

The two officials, who spoke to The Associated Press on condition of anonymity, said that the family is originally from a town in Punjab province and that the father initially moved to Saudi Arabia around three decades ago for work.

Farook had no criminal record and was not under scrutiny by local or federal law enforcement before the attack, authorities said. Friends knew him by his quick smile, his devotion to Islam and his talk about restoring cars.

They didn’t know he was busy with his wife building pipe bombs and stockpiling thousands of rounds of ammunition for the commando-style assault Wednesday on a gathering of Farook’s colleagues from San Bernardino County’s health department.

“This was a person who was successful, who had a good job, a good income, a wife and a family. What was he missing in his life?” asked Nizaam Ali, who worshipped with Farook at a mosque in San Bernardino.

Authorities said that the couple sprayed as many as 75 rounds into the room before fleeing and had more than 1,600 rounds left when they were killed. At home, they had 12 pipe bombs, tools to make more explosives and well over 4,500 rounds, police said.

On Friday morning, the owner of their rental townhome allowed reporters inside. On a living room table was a copy of the Quran. An upstairs bedroom had a crib, boxes of diapers and a computer.

The dead ranged in age from 26 to 60. Among the 21 injured were two police officers hurt during the manhunt, authorities said. Two of the wounded remained in critical condition Thursday. Nearly all the dead and wounded were county employees.

They were remembered Thursday night as several thousand mourners gathered at a ballpark for a candlelight and prayer vigil with leaders of several religions.

The soft-spoken Farook was known to pray every day at San Bernardino’s Dar Al Uloom Al Islamiyah mosque. That is where Nizaam Ali and his brother Rahemaan Ali met Farook.

The last time Rahemaan Ali saw his friend was three weeks ago, when Farook abruptly stopped coming to pray. Rahemaan Ali said Farook seemed happy and his usual self. Both brothers said they never saw anything to make them think Farook was violent.

They said Farook reported meeting his future wife online.

Abdollah reported from Washington. Contributing to this report were Associated Press writers Ken Dilanian and Eric Tucker in Washington; Zarar Khan in Islamabad, Pakistan; Brian Skoloff in Redlands, California; Mike Blood, Gillian Flaccus, Christine Armario and Justin Pritchard in Los Angeles; Holbrook Mohr in Jackson, Mississippi; Garance Burke in San Francisco; and Jason Keyser in Chicago.