Investigators believe the female shooter in California, Tashfeen Malik, pledged allegiance to Islamic State group leader Abu Bakr al-Baghdadi on Facebook, US media reported yesterday, as the probe into the deadly attack intensified.

The report comes two days after the carnage at an office year-end party in San Bernardino that left 14 people dead and 21 others wounded -- the deadliest mass shooting in the United States since the Newtown school massacre in 2012.

While authorities have publicly cautioned it is too soon to call Wednesday's carnage a terror attack, reports suggested Syed Farook, a 28-year-old US-born Muslim, and his 27-year-old Pakistani wife Malik may have been radicalized.

Reports suggested the couple may have been motivated by the IS group, which declared the creation of an Islamic "caliphate" in Iraq and Syria after seizing large swathes of territory there -- but not specifically directed to act.

"At this point we believe they were more self-radicalized and inspired by the group than actually told to do the shooting," one federal law enforcement official told The New York Times.

One US official familiar with the investigation said Malik had posted her allegiance to Baghdadi on Facebook under an account with a different name during the attack, CNN reported. MSNBC quoted another official saying the post came "just before the attack."

Officials did not explain to CNN how they knew Malik was responsible for the post. The post has since been deleted from Facebook, the Times said.

Farook and Malik were killed Wednesday in a wild firefight with police hours after the attack -- and relatives were at a loss to explain how the young couple with a baby girl could have donned black tactical gear and shot at dozens of people.

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

"I can never imagine my brother or my sister-in-law doing something like this. Especially because they were happily married, they had a beautiful six-month-old daughter," Farook's sister Saira Khan told CBS News.

Fox News reported that investigators believe there is a "very serious" possibility that Malik had radicalized her husband, who was a health inspector for San Bernardino county.

Farook attended the office party at a social services center for the disabled before apparently getting into an argument, and then returning with Malik -- both armed with assault weapons -- and opening fire.

Investigators later found thousands of rounds of ammunition at the couple's home, as well as a veritable bomb-making laboratory, with 12 pipe bomb-like devices already constructed.

Another explosive device was found at the scene of the shooting, but failed to go off.

Law enforcement officials quoted by The New York Times said the FBI was treating the shooting as a potential terrorist act, but the idea of a workplace dispute gone wrong has not been ruled out.

CNN, quoting officials, said Farook had been in contact with known terror suspects overseas. The FBI -- who were scouring cell phones and a computer hard drive of the couple -- had evidence that Farook had communicated with extremists domestically and abroad a few years ago, the Times said.

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.

Up to 3,000 people attended a vigil Thursday evening in honor of the victims, lighting candles and listening to memorial speeches.

Meanwhile members of the Arab and Muslim communities in the United States said they feared a backlash, as details emerged of the Muslim couple who shot dead 14 people in California.

"There absolutely is a fear that there could be a backlash and that's the reality we live in," said Abed Ayoub, legal and policy director for the American-Arab Anti-Discrimination Committee, a civil rights group.

Muslim officials in San Bernardino held a prayer vigil at a local mosque late Thursday to honor the victims and urged people not to link Islam with the attacks.

"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.

The American Civil Liberties Unions also urged against blaming the country's Muslim community for Wednesday's carnage.

"We musn't attribute the actions or characteristics of two individuals to an entire community," said Hector Villagra, the head of the ACLU in California.