President Barack Obama said on Thursday that the California shooting that left at least 14 people dead and wounded 21 others could be related to terrorism, but left open the potential that a workplace grievance may have been the motive.

In a sign that authorities are considering the possibility of terrorism, the US president said the FBI had taken over the investigation from local authorities. But he said the motive for the attack in San Bernardino was still unclear and could be a mix of issues.

The two suspects killed in a shootout with police on Wednesday were identified as Syed Rizwan Farook, 28, and his wife, Tashfeen Malik, 29. They had a massive arsenal of weapons on them during the shooting and at their residence in nearby Redlands, according to Jarrod Burguan, the San Bernardino police chief.

“It’s possible that this is terrorist-related but we don’t know,” said Mr Obama in an Oval Office briefing, flanked by vice-president Joe Biden, the FBI director James Comey and Loretta Lynch, attorney-general. “Our expectation is that this may take some time [to sort out]. There may be mixed motives.”

The suspects fired up to 75 rounds of ammunition at the Inland Regional Center, a state facility for people with disabilities, which was hosting a holiday party with about 75 to 80 attendees, said Mr Burguan. Authorities also found three pipe bombs combined into one device that was tied to a remote control toy car that failed to work.

The suspects shot another 76 rifle rounds in the stand-off with police that followed a hunt for them. In all, they had a stockpile of about 6,000 rounds of ammunition, while 12 pipe bombs were found at their home, in addition to hundreds of tools that could be used to construct improvised explosive devices and pipe bombs, said Mr Burguan.

“It was unspeakable, the carnage that we were seeing,” said Lieutenant Mike Madden, who was the first police responder on the scene.

Farook had worked for San Bernardino county for five years. He had attended a holiday gathering at the Inland Regional Center on Wednesday, but left under circumstances described by some witnesses as “angry”, said the police chief. He added that the suspects were not wearing body armour, as previously reported, but had tactical vests on that could hold multiple rounds of ammunition.

David Bowdich, the FBI agent in charge of the Los Angeles office, said Farook visited Pakistan and returned to the US with Malik, who had a Pakistani passport, in July 2014. He was a US citizen, while his wife was in the US on a visa.

During a forum for presidential candidates at the Republican Jewish Coalition on Thursday, Senator Ted Cruz blamed Islamic extremists for the massacre.

“All of us are deeply concerned that this is yet another manifestation of terrorism — radical Islamist terrorism here at home,” said Mr Cruz. “Coming [in] the wake of the terror attack in Paris, this horrific murder underscores that we are at a time of war. Whether or not the current administration realises it or is willing to acknowledge it, our enemies are at war with us.”

Mr Obama used the California shooting to call once again for tougher gun control laws, as he has after several mass shootings this year. Authorities said the suspects obtained their guns legally.

The San Bernardino incident is the second worst mass shooting since the 2012 massacre at the Sandy Hook elementary school in Connecticut, where 26 people were killed, including 20 children.

”Right now, it’s too easy,” said the president. “We’re going to have to search ourselves as a society to take the basic steps to make it harder — not impossible, but harder — for individuals to get access to weapons.”

Harry Reid, the Democratic Senate minority leader, will try on Thursday to force a vote on an amendment to curb gun violence, attaching it to a Republican bill to repeal Mr Obama’s signature healthcare reform legislation. Republicans have so far been successful in fighting back against stricter gun control proposals.

Mr Reid’s amendment would ban gun ownership for people who have been convicted of misdemeanours that affected the safety of abortion clinics. Last Friday, a shooting at a planned parenthood centre in Colorado left three people dead and nine wounded.

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