The FBI said Friday the mass shooting in California was being investigated as "an act of terrorism," amid reports the female assailant had pledged allegiance to the Islamic State group online.

Here is a summary of developments in the probe:

Killer couple

Wednesday's attack was carried out by a married couple, parents of a six-month-old baby girl, with no known history of violence or radicalism.

The assailants have been identified as Syed Farook, a 28-year-old US citizen who worked for the local county in environmental matters, and Tashfeen Malik, 27, a Pakistani national.

They were killed in a shootout with police hours after the attack.

The couple had left their infant daughter with Farook's mother before the attack, according to a relative.

14 dead, 21 wounded

Fourteen people were killed and 21 wounded when Farook and Malik opened fire inside the Inland Regional Center, a social services facility in San Bernardino, California.

The attack took place inside a packed conference room rented by the county's public health department for a year-end event, which Farook had been attending.

Authorities identified the victims as six women and eight men ranging in age from 26 to 60. All but two were county employees and co-workers of Farook.

Act of terror?

The FBI said Friday the mass shooting is now being investigated as terrorism.

"We have uncovered evidence that has led us to learn of extensive planning," the assistant FBI director in charge of the Los Angeles office, David Bowdich, told reporters.

"There's a number of pieces of evidence that has essentially pushed us off the cliff to say we are now investigating this as an act of terrorism."

FBI Director James Comey said, however, that there was no indication the couple were "part of an organized larger group or part of a cell."

Authorities have also been looking into a work-related cause, following reports Farook may have snapped at his office party when a religious discussion got out of hand.

One witness said he stormed out of the event, leaving his jacket on his seat, and returned shortly after heavily armed, dressed in black tactical gear and a mask -- and accompanied by his wife.

Allegiance to Islamic State group

Malik is believed to have pledged allegiance to the Islamic State group in a Facebook post made under another name, around the time of the attack, according to media reports.

The FBI has confirmed it is looking into the supposed pledge.

"I'm aware of the Facebook posting," Bowdich told reporters. "And yes, there was a pledge of allegiance."

Pakistan, Saudi Arabia

Investigators are looking into several foreign trips made by Farook prior to July 2014, according to the FBI.

He notably visited Saudi Arabia. Federal investigators have also suggested he traveled to Pakistan, but that is denied by the family's lawyer.

His wife held a Pakistani passport and traveled to the United States on a K1 fiancee's visa.

The couple is believed to have made a pilgrimage to Mecca in the summer of 2014, during which they married. Malik has spent time living in Saudi Arabia.

Heavily armed

The couple were armed with assault rifles and handguns -- all legally purchased. They fired dozens of rounds at the Inland Regional Center, before fleeing in a black SUV.

Explosives rigged to a remote-controlled car were also found at the scene of the carnage, but the device failed to go off.

Police said 1,600 rounds of ammunition were found on the couple and in their car following the police shootout in which they died.

About 5,000 additional rounds, 12 pipe bomb-type explosive devices and bomb-making material were found at the couple's home.