The two attackers who killed 14 people at a banquet fired as many as 75 rifle rounds at the scene, left behind three rigged-together pipe bombs with a remote-control device that apparently malfunctioned and had 1,600 more bullets with them when they were gunned down in their SUV.
At their home, they had 12 pipe bombs, tools for making more such explosives and more than 3,000 rounds of ammunition, Police Chief Jarrod Burguan said in a grim morning-after inventory that suggested Wednesday's bloodbath could have been far bloodier.
Syed Rizwan Farook, a 28-year-old county restaurant inspector, and his wife or fiancee, Tashfeen Malik (27), murdered 14 people and seriously wounded more than a dozen in the attack at a social service centre for the disabled.
The couple were gunned down hours later a few miles away in a shootout with police.
As the FBI took over the investigation, authorities were trying to learn why the couple left behind their six-month-old daughter and went on the rampage - the nation's deadliest mass shooting since the Newtown, Connecticut, school tragedy three years ago that left 26 children and adults dead.
President Barack Obama said it was "possible this was terrorist-related" but that authorities were unsure. He raised the possibility that it was a workplace dispute or that mixed motives were at play.
Farook was born in the US to a Pakistani family, raised in Southern California and had been a San Bernardino County employee for five years. Chief Burguan said he had no information on Malik's background.
The pair attacked the centre, about 60 miles east of Los Angeles, at around 11am, opening fire in a conference area county health officials had rented for an employee banquet.
"They came prepared to do what they did, as if they were on a mission," Chief Burguan said.
Co-worker Patrick Baccari said that when the shooting started, he took refuge in a bathroom and suffered minor wounds from shrapnel.
Mr Baccari said Farook travelled to Saudi Arabia, was gone for about a month, returned with a wife and grew a beard.
"We don't know the motives. Is it work, rage-related? Is it mental illness? Is it extreme ideology?" said Hussam Ayloush, executive director of the Council on American-Islamic Relations.
Co-workers said Farook was a devout Muslim, but did not talk about religion at work.
Farhan Khan, who is married to Farook's sister, condemned the violence and said he had "no idea" why Farook would kill.
About four hours after the late-morning carnage, police hunting the killers riddled a black SUV with gunfire in a shootout two miles from the social services centre.