SAN BERNARDINO, Calif. -- The FBI said Friday that it is officially investigating the mass shooting in California as an act of terrorism, while a federal law enforcement source confirmed to CBS News that the woman who helped her husband carry out the attack had pledged allegiance to the Islamic State of Iraq and Syria's leader online.
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.
The source confirmed to CBS News that Malik pledged her allegiance to ISIS leader Abu Bakr al-Baghdadi in an online posting, CBS News senior investigative producer Pat Milton reports.
A law enforcement source told CBS News that Malik's posting was under a fake name, Milton reports. The source said the pledge does not mean the attack was ordered or directed by ISIS.
A Facebook official says Malik praised ISIS' leader 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.
A Facebook spokesperson confirmed to CBS News it "identified and removed the profile for violating our community standards."
"We don't allow people to praise acts of terror or promote terrorism," the spokesperson said. "We also work aggressively to ensure that we do not have terrorists or terror groups using the site."
Facebook is cooperating with law enforcement, according to the spokesperson.
Malik, 27, was a Pakistani who grew up in Saudi Arabia and came to the U.S. in 2014 on a fiancee visa. Farook, a 28-year-old restaurant health inspector for the county, was born in Chicago to Pakistani parents and raised in Southern California.
A source told CBS News that the pair were looking at ISIS propaganda online.
Two law enforcement sources told CBS News that Farook had been in contact with an individual in the U.S. with suspected ties to terrorism who was known to the FBI, Milton reports. According to the sources, Farook was in communication with people overseas who have links to terrorism.
The sources told CBS News that investigators are focused on Farook's domestic and overseas contacts to try to determine to what extent he was influenced by those individuals, Milton reports.
FBI Director James Comey on Friday said the investigation indicates that the attackers were radicalized and might have been inspired by terrorist groups overseas.
"So far we have no indication that these killers are part of an organized larger group or part of a cell," Comey said, adding that there's "no indication that they're part of a network."
The FBI had 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.
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, ISIS 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 ISIS 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 AP 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 worshiped 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.