© Jim Wilson/The New York Times A member of an F.B.I. evidence response team enters a townhome in Redlands, Calif., linked to the shooting rampage.
Malik posted under a different name, a US official familiar with the investigation told CNN.
The post was later removed, apparently by Malik herself, the Los Angeles Times reported, citing law enforcement officials. However, FBI technicians were able to recover it.
The post appeared "just before the attack," a law enforcement source told NBC News.
If confirmed, the finding could be a "game changer" in the investigation, two officials told Reuters.
Malik, 27, and her husband, 28-year-old Syed Rizwan Farook, are suspected in the Wednesday attack that killed 14 people and injured 21 more at a holiday party for the San Bernardino County Department of Public Health, where Farook worked as an environmental specialist.
Malik was a Pakistani native who was living in Saudi Arabia when she met Farook, who was born in Chicago, Illinois and raised in California. She entered the United States on a K-1, or "fiancée," visa, meaning the couple had to marry within 90 days of her arrival.
Investigators are trying to determine whether Malik radicalized her husband. He had been different ever since he returned from meeting her in Saudi Arabia, Christian Nwadike, who worked with Farook for five years, told CBS.
"I think he married a terrorist," Nwadike said.
Investigators are also looking into an argument between Farook and a coworker who is reported to have denounced the "inherent dangers of Islam," a US government source told Reuters.
The couple destroyed computer hard drives and other electronic devices before the shooting, the source added. Investigators are still hoping to recover information from the couple's computers and cellphones that will reveal whether they had browsed jihadist websites, or had contact with militant groups, before the attack at the Inland Regional Center on Wednesday.
Comment: Do police have video footage of them destroying the drives? 1) We cannot assume these people were working alone. If they were not, then 2) someone else may have destroyed the gear. The fact that possibilities like this are not even mentioned show there is no intention of determining what actually happened; there is only the narrative the authorities want presented. Talk about Keystone Cops!
Farook had several profiles on Muslim-oriented dating sites, ABC News reported. In one, he described himself as part of a "religios [sic] but modern family of 4" who enjoyed "doing target practice with younger sister and friends" in his backyard.
"I try to live as a good Muslim," Farook wrote in another. "Looking for a girl who has the same outlook, wear hijab, but live the life to the fullest."
Both devout Muslims, Farook and Malik met online in 2014. The couple married on August 16 in Riverside, according to a marriage certificate obtained by ABC News. They had a six-month-old daughter, police said. The baby was staying with Farook's mother during the shooting.
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 for the massacre.
"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?"
Comment: Good question...
Malik was from the Layyah district in Pakistan's southern Punjab province, but her family moved to Saudi Arabia about 25 years ago, two Pakistani officials told Reuters. She returned home "five or six years ago" to complete a degree in pharmacy from Bahauddin Zakariya University in Multan.
Her father had become considerably more conservative since moving to Saudi Arabia, Malik's uncle Javed Rabbani told Reuters, adding that Pakistani intelligence officials had contacted him as part of the investigation.
"I only found out about this tragedy today when some intelligence officials contacted me to ask me about my links with Tashfeen," Rabbani said. "I had heard in the news that this tragedy had taken place but I could never even imagine that it would be someone from my family. Of course, we are in shock."
Comment: Let's get this straight: She pledges allegiance to to ISIS under a different name then deletes said pledge, right before heading off on a shooting spree that will likely end in her own death? That doesn't sound fishy to anyone in law enforcement? Is no one even raising the possibility that someone hacked her account? Guess not - that would require a few too many neurons.