California shooter had been in contact with 'Islamic extremists'
California gunman Syed Rizwan Farook had been in contact with known Islamic extremists on social media, a US intelligence source has said.
And Farook and his wife had enough bullets and bombs to slaughter hundreds when they launched their deadly attack on a party at a social services centre for the disabled, police said.
The details emerged as investigators tried to determine whether Wednesday's rampage that killed 14 people was terrorism, a workplace grudge or a combination of factors.
The husband-and-wife killers were not under FBI scrutiny before the massacre, a US official said.
Wearing black tactical gear and wielding assault rifles, Farook, 28, a restaurant inspector, and wife Tashfeen Malik, 27, sprayed as many as 75 rounds into a room at the centre in San Bernadino, where about 75 of Farook's co-workers had gathered. 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 gun battle in which they fired 76 rounds, while 23 law enforcement officers unleashed about 380, police said.
Police chief Jarrod Burguan offered a grim inventory today that suggested the bloodbath could have been far worse.
At the centre, the couple left three pipe bombs with a remote-control detonating device that apparently malfunctioned and had more than 1,600 rounds of ammunition remaining when police killed them in their rented SUV, Mr Burguan said.
At a family home in the nearby town of Redlands, they had 12 pipe bombs, tools for making more, and more than 3,000 additional rounds of ammunition.
"We don't know if this was workplace rage or something larger or a combination of both," US attorney general Loretta Lynch said in Washington. "We don't know the motivation."
Investigators are trying to determine whether Farook, who was Muslim, became radicalised and, if so, how, as well as whether he was in contact with any foreign terrorist organisation, the intelligence source said.
The official said Farook had been in touch on social media with extremists who were being watched by the FBI.
Another official said the FBI was treating the attack as a potential act of terror but had reached no conclusion. The official said Farook's contact was with "people who weren't significant players on our radar", dated back some time, and had no immediate indication of a recent surge in communication.
Rita Katz, director of the SITE Intelligence Group, an organisation that tracks and analyses extremists, said it had not found any connection between Farook and jihadi groups. But she also said some of Farook's social media posts seemed to have been deleted before the attack.
The San Bernadino massacre was America's deadliest mass shooting since 2012, when 26 children and adults were killed at Sandy Hook Elementary School in Newtown, Connecticut.
In San Bernardino, a Southern California city of 214,000, the victims' ages ranged from 26 to 60. A further 21 people were injured, including two police officers. Two of the wounded remain in a critical condition.
Nearly all the dead and wounded were local authority employees.
Farook, who had no known criminal record, 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 totalling four years since 2010. The Saudi embassy said he travelled to Saudi Arabia in the summer of 2014 for nine days.
Malik came to the US in July 2014 on a Pakistani passport and a fiancee 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 August 16 last year in nearby Riverside County, according to their marriage licence. Both listed their religion as Muslim. The couple had a six-month-old daughter who they left with relatives on Wednesday morning before the shooting.
Farook was a devout Muslim who prayed every day and recently memorised the Koran, according to brothers Nizaam and Rahemaan Ali, who attended Dar Al Uloom Al Islamiyah mosque in San Bernardino with him.
Rahemaan Ali said he last saw Farook three weeks ago, when he abruptly stopped going to the mosque. He said Farook seemed happy and his usual self and the brothers never saw a violent side.
Authorities said Farook bought two handguns used in the massacre legally and their two assault rifles were legally bought by someone else. Authorities did not say how the rifles got into the attackers' hands.
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.
Ms Stephens said she called Mr Thalasinos while he was talking with Farook at work Mr Thalasinos told her Farook "doesn't agree that Islam is not a peaceful religion".
Work colleague Patrick Baccari said until the rampage, Farook showed no signs of unusual behaviour and was a reserved young man.
Lieutenant Mike Madden, one of the first police officers to reach the room where the massacre took place, said the carnage was "unspeakable" and the scene overwhelming.
He described 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.
Muslims who gathered for a vigil at one of Southern California's largest mosques condemned the massacre.
About 200 people held the night vigil at the mosque in Chino, about 24 miles away from the shooting scene to pray and send a message that Ahsan Khan described as "love for all, hatred for none".
Mr Khan and other leaders of the Ahmadiyya Muslim Community described the shooting as deplorable and horrific.
Group spokesman Amjad Mahmood Khan warned that no one yet knew the motives of the killers and could not presume it was faith-driven. But he added that the "radicalisation of Muslim youth is a problem".