Gunwoman in San Bernardino Pledged Allegiance to Islamic State Leader on Facebook

Editor's note: FBI Director James Comey clarified on December 16 that Tashfeen Malik expressed support for "jihad and martyrdom" in private communications but never did so on social media.

Two days after two shooters reportedly gunned down 14 people in San Bernardino, California, investigators said that the wife of the young couple suspected in the shooting posted a pledge of allegiance to the leader of the Islamic State militant group on Facebook, and then deleted it shortly before opening fire on guests at a holiday party.

Tashfeen Malik, 27, allegedly used a different Facebook account under an assumed name to post a message of allegiance to Islamic state Caliph Abu Bakr al-Baghdadi on the day she and her husband Syed Rizwan Farook, 28, attacked the Inland Regional Center social services agency in San Bernardino Wednesday, a US law enforcement official told Reuters Friday.

The official did not disclose further details of the post.

An unidentified law enforcement official also told CNN: "This is looking more and more like self-radicalization."

The new information comes as authorities work to piece together information and a motive for the shooting that also left 21 people wounded. The Facebook post is the first piece of evidence to solidify authorities' suggestions that the attack was possibly motivated by extremist ideology.

Malik and Farook were killed in a shootout with police shortly after the attack, leaving behind a 6-month-old daughter whom they left with Farook's mother on Wednesday morning after telling her they were going to run an errand.

Farook's brother-in-law, Farhan Khan, told NBC News he had begun legal proceedings to adopt the girl and was "very upset and angry" at Farook.

"You left your 6-month-old daughter," Khan said. "In this life some people cannot have kids. God gave you a gift of a daughter. And you left that kid behind … What did you achieve?"

Related: What We Know So Far About the Suspects in The San Bernardino Shooting

Investigators are reviewing the couples' computers and cellphones to see if they had browsed jihadist websites or had contact with militant groups, officials in Washington familiar with the investigation told Reuters.

Police said the couple had two assault-style rifles, two semi-automatic handguns and 1,600 rounds of ammunition in their vehicle, and found 12 pipe bombs in in the couple's home.

Authorities are still working to put together a broader picture of both Malik, who was born in Pakistan and had spent time in Saudi Arabia before coming to the United States, and Farook, who is an American citizen, born to parents who immigrated from Pakistan.

Farook worked as an environmental inspector and had attended the office party at the Inland Regional Center with colleagues on Wednesday, police said. He had reportedly angrily left the party after an argument and returned with Malik— both wearing tactical vests that could hold weapons and ammunition — to unleash the attack.

Colleagues said that Farook was quiet and withdrawn — a man who loved fixing up cars and was a devout Muslim. They told the LA Times they were aware Farook had recently returned from Saudi Arabia with a bride he had met online and had since grown a beard, but that there were no other hints of the violence to come. According to police, the pair had entered the US in July of 2014, but investigators declined to say for how long and where they had been traveling.

"It just doesn't make sense for these two to be able to act like some kind of Bonnie and Clyde or something," Farook family attorney David S. Chesley told CNN. "It's just ridiculous. It doesn't add up."

The latest mass shooting has been called the worst gun violence in the nation since the December 2012 shooting at Sandy Hook Elementary School in Newtown, Connecticut, and has reignited the debate on gun control laws in the US.

Two of the guns the suspects used were purchased legally, according to authorities.

The Islamic State claimed responsibility for the November 13 coordinated shooting and bombing attacks in Paris that killed 130 people and injured hundreds. The group has called on its supporters around the world to strike targets in the West.

Watch VICE News' documentary The Islamic State: