SAN BERNARDINO, United States (AFP) - A young couple behind a mass shooting in California that left 14 dead had amassed a huge arsenal and carefully planned their attack, authorities said Thursday, refusing to rule out terrorism.
The suspects - Syed Farook, 28, and his 27-year-old wife Tashfeen Malik - were killed in a shootout with police after Wednesday's rampage at a holiday party in San Bernardino, about an hour's drive east of Los Angeles.
Police revised upwards to 21 the number of people injured in the attack - the deadliest in the United States since the 2012 massacre at a school in Connecticut that left 26 people dead, including 20 children.
San Bernardino police chief Jarrod Burguan said Farook and his wife had fired between 65 and 75 rounds during the attack at a social services center that entailed "a degree of planning."
Explosives rigged to a remote-controlled car were also found at the scene of the carnage, but the device failed to go off, he said.
He said 1,600 rounds of ammunition were found on the couple and in their car following the police shootout in which they died.
About 5,000 additional rounds, 12 pipe bomb-type explosive devices and bomb-making material were found at the home the couple shared.
"Nobody just gets upset at a party, goes home and puts together that kind of elaborate scheme," Burguan said, after indications that Farook had attended the party organised by the health department and left after an apparent dispute, only to return a short time later with Malik.
"There was some planning that went into this."
US President Barack Obama cautioned the motive for the attack was not yet known, but a terror attack could not be ruled out.
"At this stage, we do not yet know why the terrible event occurred," said Obama, who has repeatedly called on the Republican-controlled Congress to pass tougher gun control measures.
"It is possible that this was terrorist-related. But we don't know. It's also possible that this was workplace-related," the President said.
The Federal Bureau of Investigation (FBI) also cautioned that it was "way too early" to speculate on the motive.
But media reports said Farook, who is US-born and worked for the local county as an environmental inspector, had met his Pakistani-born wife during a trip to Saudi Arabia in 2013 and had become radicalised.
Burguan said Farook had brought his wife back to the United States in July of last year, and that both had had no brushes with the law.
CNN quoted law enforcement officials as saying he had been in touch via social media and by phone with international terror subjects known to the FBI.
The couple, who dropped off their baby daughter with Farook's mother shortly before the rampage, were dressed in black military-style gear and carried assault weapons and semi-automatic handguns as they burst into an auditorium rented out for the holiday party at Inland Regional Center.
Farook's brother-in-law, Farhan Khan, said he had no clue what prompted the carnage.
"I am in shock that something like this could happen," a visibly shaken Khan told a press conference organised by the Council on American-Islamic Relations (Cair), which denounced the killings.
Local media quoted one of Farook's co-workers as saying he and his wife seemed happy and appeared to be "living the American dream."
"He never struck me as a fanatic, he never struck me as suspicious," Griselda Reisinger, who worked with Farook until changing jobs in May, told the Los Angeles Times.
Hussam Ayloush, executive director of Cair in Los Angeles, said the couple had been married for two years and "had a good life."
"We don't know the motive yet. This could be a workplace rage, this could be a result of some of instability, mental instability, or it could be some twisted ideological belief," Ayloush told NPR news.
Witnesses at the California scene recounted barricading themselves in offices and hiding as the sound of gunfire erupted.
Mark Stutte said his daughter was attending the party and called him terrified while hiding in a restroom as gunshots rang out in the background.
"It was really, really super scary," he told local TV, as he wept. "I'm far away. I couldn't do anything for her."
According to the site Mass Shooting Tracker, the latest attack brings to 352 the number of mass shootings in the United States so far this year. A mass shooting is defined as four or more people shot in one incident.
"Violence like this has no place in this country and in this nation," US Attorney-General Loretta Lynch said Thursday.