California shooters had massive arsenal: police

There were competing theories as to what drove US-born Syed Farook, 28, described as a committed Muslim, and his 27-year-old wife Tashfeen Malik to spray gunfire at a holiday party in San Bernardino, about an hour's drive east of Los Angeles.

The attack, which left 21 people wounded, was 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 -- who dropped off their six-month-old daughter with Farook's mother shortly before the killings -- fired about 75 rounds during the attack at a disabilities center that required "a degree of planning."

But the latest mass shooting in the United States could have been worse, after Burguan revealed that explosives rigged to a remote-controlled car were found at the scene of the carnage but the device failed to go off.

Another 1,600 rounds of ammunition were found on the couple and in their bullet-riddled black SUV after a police chase and shootout in which they died. Two police officers were wounded in the rapid exchange of gunfire on a quiet residential street, though neither seriously.

About 5,000 additional rounds, 12 pipe bombs and bomb-making material were also 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 organized by the health department and left following a dispute, only to return a short time later with Malik.

He said the duo were dressed in black military-style gear and carried assault weapons and semi-automatic handguns when they raided the Inland Regional Center shortly before lunch time Wednesday.

President Barack Obama, who ordered flags to be flown at half-staff until Monday, cautioned the motive for the carnage 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 FBI also cautioned that it was "way too early" to speculate on the motive.

Dressed in black

Acquaintances told AFP that Farook, who worked for the local county as an environmental inspector, married his Pakistan-born wife last year during a trip to Mecca, in Saudi Arabia.

Burguan said the couple had no criminal record. But CNN quoted law enforcement officials as saying Farook had become radicalized and had contact with known terrorism suspects overseas.

An imam at the local mosque that Farook attended denied that and said there was no indication he held extremist views.

A fellow worshipper at the Dar Al Uloom Al Islamiyah mosque said Farook used to pray there two to three times a week, but had not been seen for about three weeks.

Another acquaintance said Farook met Malik online and they married in Mecca last year during the holy Muslim month of Ramadan.

"I saw her (Malik) a couple times, but never met her," said Nizaam Ali, a university student. "She was covered from head to toe in black."

Although officials have yet to release the identity of the victims, some names have begun emerging on social media.

"My husband, Nicholas Thalasinos, was killed in the shooting," wrote Jennifer Thalasinos, a teacher, on her Facebook page.

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.

Local Muslim officials in San Bernardino said a prayer vigil would be held at the local mosque later Thursday to honor the victims.

"We condemn this senseless and horrific act of violence in the strongest possible terms," said Ahsan Khan, president of the Ahmadiyya Muslim Community chapter in Los Angeles.

"Our community has been in San Bernardino County for nearly three decades, and yet have never seen such depravity."

Abed Ayoub, legal and policy director at the American-Arab Anti-Discrimination Committee (ADC), said there was fear the attacks would lead to a backlash against the Muslim community.