The details emerged as investigators tried to determine whether the rampage that left 14 people dead was terrorism, a workplace grudge or some combination.
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The husband-and-wife killers were not under FBI scrutiny before the massacre, said a second U.S. official, who likewise was not authorized to discuss the investigation and spoke on condition of anonymity.
Wearing black tactical gear and wielding assault rifles, Farook, a 28-year-old county restaurant inspector, and his wife, Tashfeen Malik, 27, sprayed as many as 75 rounds into a room at a social service center for the disabled, where about 75 of Farook's co-workers had gathered Wednesday morning. Farook had attended the event but slipped out and returned in battle dress.
Four hours later and two miles away, the couple died in a furious gunbattle in which they fired 76 rounds, while 23 law officers unleashed about 380, police said.
On Thursday, Police Chief Jarrod Burguan offered a grim inventory that suggested Wednesday's bloodbath could have been far worse.
At the social service center, the couple left three rigged-together pipe bombs with a remote-control detonating device that apparently malfunctioned, and they had more than 1,600 rounds of ammunition remaining when police killed them in their rented SUV, Burguan said.
At a family home in the nearby town of Redlands, they had 12 pipe bombs, tools for making more, and over 3,000 additional rounds of ammunition, the chief said.
"We don't know if this was workplace rage or something larger or a combination of both," Attorney General Loretta Lynch said in Washington, echoing President Barack Obama. "We don't know the motivation."
Investigators are trying to determine whether Farook, who was Muslim, became radicalized - and, if so, how - as well as whether he was in contact with any foreign terrorist organization, said the U.S. intelligence official, who was not authorized to discuss the matter publicly and spoke on condition of anonymity.
The same official said Farook had been in touch on social media with extremists who were under FBI scrutiny.
The second U.S. official said the FBI was treating the attack as a potential act of terror but had reached no conclusion that it was. The official said Farook's contacts online did not involve any significant players the FBI knew of and dated back some time, with there was no immediate indication of a recent surge in communication.
The official cautioned that such contact by itself doesn't mean someone is a terrorist.
Rita Katz, director of SITE Intelligence Group, an organization that tracks and analyzes extremists, said it hasn't found any connection between Farook and jihadi groups. But she also said that some of Farook's social media posts seem to have been deleted before the attack.
Wednesday's rampage was the nation's deadliest mass shooting since 2012, when 26 children and adults were slain in Newtown, Connecticut.
In San Bernardino, a Southern California city of 214,000, the victims ranged in age from 26 to 60. A further 21 people were injured, including two police officers, authorities said. Two of the wounded remained in critical condition Thursday.
Nearly all the dead and wounded were county employees.
Authorities said the attack was carefully planned.
"There was obviously a mission here. We know that. We do not know why. We don't know if this was the intended target or if there was something that triggered him to do this immediately," David Bowdich, assistant director of the FBI's Los Angeles office, said as the bureau took over the investigation.
Farook has no known criminal record, Burguan said. He was born in Chicago to a Pakistani family, raised in Southern California and worked at San Bernardino County's Department of Public Health for two stints totaling four years since 2010, according to authorities and acquaintances. The Saudi Embassy said he traveled to Saudi Arabia in the summer of 2014 for nine days.
As for Malik, she came to the U.S. in July 2014 on a Pakistani passport and a fiance visa, authorities said. To get the visa, immigrants submit to an interview and biometric and background checks - screening intended to identify anyone who might pose a threat.
They were married on Aug. 16, 2014, in nearby Riverside County, according to their marriage license. Both listed their religion as Muslim.
The couple had a 6-month-old daughter who they dropped with relatives Wednesday morning before the shooting.
Adam Lankford, a criminal justice professor at the University of Alabama, said his review of mass public shootings in the U.S. indicates this is the first one in recent history involving a male-female team.
Farook was a devout Muslim who prayed every day and recently memorized the Quran, according to brothers Nizaam and Rahemaan Ali, who attended Dar Al Uloom Al Islamiyah mosque in San Bernardino with Farook.
Rahemaan Ali said he last saw Farook three weeks ago, when he abruptly stopped going to the mosque. Ali said Farook seemed happy and his usual self, and the brothers never saw a violent side.
"He never ever talked about killing people or discussed politics, or said that he had problems at work," Rahemaan Ali said. "He always had a smile on his face."
Federal authorities said Farook legally bought two handguns used in the massacre and their two assault rifles were legally bought by someone else. Authorities did not say how the rifles got into the attackers' hands.
Farook apparently was no stranger to guns.
A profile on a matchmaking website for South Asians that matched Farook's name, California hometown, county health job and Muslim faith said his interests included target shooting in his backyard. Though the date of the posting was not clear, it listed his age as 22 so it could have been six years old.
Two weeks ago, Farook and one of the co-workers he killed, 52-year-old Nicholas Thalasinos, had a heated conversation about Islam, according to Kuuleme Stephens, a friend of the victim.
Stephens said she happened to call Thalasinos while he was talking with Farook at work. She said Thalasinos told her Farook "doesn't agree that Islam is not a peaceful religion."
As for workplace conflicts, co-worker Patrick Baccari said that until the rampage, Farook showed no signs of unusual behavior and was a reserved young man.
Baccari said he was sitting at the same table as Farook before Farook suddenly disappeared, leaving his coat on his chair. Baccari was in the bathroom when the shooting started; he suffered minor wounds from shrapnel slicing through the wall.
One of the first officers to reach the room said the carnage was "unspeakable" and the scene overwhelming - the smell of gunpowder, the wails of the injured, the blood, fire sprinklers pumping and fire alarms blaring. All in a room with a Christmas tree and decorations on every table.
"As we entered," said San Bernardino Police Lt. Mike Madden, "the situation was surreal."
SHOOTING SUSPECT BORN IN CHICAGO
Syed Farook was born in June 1987 at Cook County Hospital in Chicago and has lived at several different addresses in the Chicago area.
Farook and his wife Tashfeen Malik were married in California in August 2014. But before the wedding, he traveled to Saudi Arabia to bring her back to this country, specifically to Chicago. On July 16, 2014, he flew from Chicago to Heathrow to Jedda, Saudi Arabia. He then flew back in reverse order, with his fiance on July 27, arriving back in Chicago.
The couple had a 6-month-old child, who they dropped off with relatives before their deadly rampage.
Some local Muslims say they are concerned about backlash, though Farook's motive is still unclear.
"This particular incident is not indicative of the religion, it's indicative of extremism," said Tayyib Rashid, Ahmadiyya Muslim spokesperson.
Farook's family lived at one time in a townhome in Niles. The current resident says the FBI showed her pictures of the father, but she says he moved seven years ago. Apparently about the same time, his father's wife filed a domestic violence complaint.
According to the domestic violence petition that Farook's mother filed against his father, she accused him of being mentally abusive, screaming, throwing things and being an alcoholic. She said that he was particularly very difficult with his children, including Farook.
Farook's driver's license was registered to California, which is where he went to high school and where his life ended in a hail of gunfire.
Mass shootings have become disturbingly common in the United States.
After the bloodshed in California many people are worried about a similar incident happening here in Chicago.
There is currently no credible threat to the Chicago area. But response training has already taken place.
The Illinois Emergency Operations Center has been training for how to best respond to a mass shooting. They haven't been activated, but they are in contact with federal investigators just in case.
Wednesday night at the University of Chicago, former Undersecretary of Defense for Intelligence Michael Vickers was invited to speak about the modern Middle East. But he weighed in on the San Bernardino shooting, saying it looked like it involved a lot of planning and coordination.
Arnette Heintze of Hillard Heintze, an investigation and security risk management company, said as a society, we are in a new phase, a new area of concern.
"The odds of you being in a situation like this are very remote. But when it happens, it's tragic, personal. Our hearts go out to the victims and their families," Heintze said.
"We just have to evolve as the threat evolves," Vickers said.
WLS-TV and the Associated Press contributed to this report.