San Bernardino shooter pledged allegiance to IS, say US investigators

A woman who carried out as mass shooting with her husband in California pledged allegiance to the Islamic State on Facebook, US federal investigators have said.

In a post now deleted from Facebook, Tashfeen Malik swore allegiance to the group, before she and her husband Syed Rizwan Farook attacked a Christmas party at a social services centre for the disabled.

The FBI said it had launched an investigation into an "act of terrorism" after uncovering to evidence of "extensive planning".

Fourteen people were killed and 21 wounded in the attack on Wednesday evening at the Inland Regional Centre in San Bernardino, near Los Angeles. Both Malik, 27, and Farook, 28, were killed by police shortly after.

The federal investigators said that despite the pledge to IS, they did not believe the shooting was carried out on orders from Syria.

"At this point we believe they were more self-radicalised and inspired by the group than actually told to do the shooting," one of the officials said.

Another was quoted by MSNBC as saying Malik's message was posted on Facebook "just before the attack".

San Bernardino police chief Jarrod Burguan said on Thursday that Farook had attended the party but left early "under circumstances described as angry or something of that nature".

Police said Farook and his wife had used variants of the AR-15 semi-automatic rifle and two handguns in the attack, and had fired between 65 and 75 bullets. 

Police also found an explosives-rigged remote-controlled car at the scene, and 1,600 rounds of ammunition were found on the couple and in their car after they were shot dead.

About 5,000 additional rounds, 12 pipe bomb-type explosive devices and bomb-making material were found at the home they shared, police said. 

"Nobody just gets upset at a party, goes home and puts together that kind of elaborate scheme," Burguan said.

The attack was the deadliest mass shooting in the US since the Sandy Hook Elementary School massacre three years ago when 20-year-old Adam Lanza killed 20 children and six staff.

Farook's brother-in-law, Farhan Kahn, came forward at the press conference to voice his shock at the mass murder.

"I have no idea why he would do that," a visibly shaken Khan said, adding that he had last spoken with Farook about a week ago.

"I am in shock that something like this could happen. I am very sad that people lost their lives."

Malik was born in Pakistan and had recently lived in Saudi Arabia. Farook worked for San Bernadino's environmental health department and was born in Illinois to Pakistani immigrants.

There are no signs that the couple were part of a larger group, FBI Director James Comey said Friday.

"So far we have no indication that these killers are part of an organised larger group or part of a cell. There's no indication that they are part of a network," Comey said, after US officials revealed they were investigating the shooting as an "act of terrorism."

Police said they believed that the couple were able to buy their weapons legally despite California having some of the most restrictive gun laws in the United States.