WASHINGTON: Tashfeen Malik, the female shooter in the California rampage that left 14 dead, is believed to have pledged allegiance to Islamic State leader Abu Bakr al-Baghdadi, CNN reported on Friday, citing three US officials.

One of the officials said Malik had pledged allegiance to al-Baghdadi in a posting on Facebook under an account that used a different name, CNN said.

Federal Bureau of Investigation (FBI) agents have taken charge of the investigation into Wednesday's mass shooting in San Bernardino and were combing through evidence, including cell phones and a computer hard drive, to determine what prompted Syed Farook, 28, and his 27-year-old Pakistani wife Tashfeen Malik to carry out the rampage that also left 21 people wounded.

Know more: California killing: Female assailant was from Pakistan, CAIR claims

Law enforcement officials quoted by The New York Times said the FBI was treating the shooting as a potential terrorist act, but the agency was far from concluding it was.

CNN, quoting officials, also said Farook was in contact with known terror suspects overseas and had become radicalised after marrying Malik in Saudi Arabia last year, although an imam at a local mosque he attended said Farook showed no signs of that.

United States (US) President Barack Obama, who ordered flags to be flown at half-staff until Monday, said a terror attack could not be ruled out but cautioned against jumping to conclusions.

"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, after a string of mass shootings across the US in recent years.

"It is possible that this was terrorist-related, but we don't know. It's also possible that this was workplace-related."

The attack was the deadliest mass shooting in the US since the 2012 assault on an elementary school in Connecticut that left 26 people dead, including 20 children.

Intended target?

Some of the reasons pushing authorities to believe Wednesday's shooting may be terror-related was the astonishing arsenal the couple had amassed, their foreign travels, and the fact that they appeared to have meticulously planned the attack.

"There was obviously a mission here," David Bowdich, the assistant FBI director in charge of the Los Angeles office said, in the wake of the killings at a holiday party for county employees at a social services centre.

"We don't know if this was the intended target or if there was something that triggered him to do this immediately."

San Bernardino police chief Jarrod Burguan said Farook and his wife — who dropped off their six-month-old daughter with Farook's mother shortly beforehand — fired about 150 bullets inside the Inland Regional centre and during a subsequent shootout with police that left both dead, after a huge manhunt.

He said investigators had found an additional 5,000 rounds of ammunition at the couple's home along with 12 pipe bombs and bomb-making material.

"Nobody just gets upset at a party, goes home and puts together that kind of elaborate scheme," Burguan said, referring to indications that Farook had attended the party and left following a dispute, only to return a short time later with Malik.

'Unspeakable carnage'

The duo were dressed in black military-style gear and carried assault weapons and semi-automatic handguns when they raided the party where about 80 people had gathered shortly before lunchtime.

Authorities identified the victims as six women and eight men ranging in age from 26 to 60.

All but two were county employees and colleagues of Farook, who worked as an environmental inspector for the health department.

Lieutenant Mike Madden, one of the first police officers to respond to the shooting, said he came upon a "surreal' scene on entering the building.

"It was unspeakable," he said. "The carnage that we were seeing, the number of people who were injured and unfortunately already dead and the pure panic on the face of those individuals still alive."

'We are not afraid'

Several vigils, including one at a local mosque, were held in San Bernardino on Thursday evening. "This is a tragedy but we must show that we are not afraid," said Dorothy Andrews, 74, who joined several thousand people who turned out at the city's San Manuel Stadium.

Acquaintances told AFP that Farook did not seem to have extremist views and was living "the American dream" with his wife and baby daughter.

"He was married, he had a daughter and last year he made $77,000," said Gasser Shehata, 42, who attended the same local mosque as Farook. "He had everything to be happy."

Another 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.

According to the site Mass Shooting Tracker, the latest attack brings to 352 the number of mass shootings in the US so far this year.

A mass shooting is defined as four or more people shot in one incident.

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.

"We need to stay cautious given the atmosphere and what happened in Paris a few weeks ago and the fallout from that and the continued rhetoric," Ayoub told AFP, referring to the recent terror attacks in France that left 130 dead.

Read: At least 128 dead in Paris terror attacks, IS claims responsibility