The terror group claims Malik, 29, and her husband, Syed Rizwan Farook, 28, were "supporters
" of ISIS. A law enforcement official said it appeared last week's San Bernardino attack may have been inspired by ISIS, but none of the three officials said the group directed or ordered the attack.
The post, declaring her loyalty to Abu Bakr Al-Baghdadi, was made on an account with a different name, although officials did not explain how they knew Malik made the post.
Malik and Farook carried out Wednesday's shooting at the Inland Regional Center that also injured 21 people, authorities said. They died later in a police shootout on a San Bernardino street.
"This is looking more and more like self-radicalization," a law enforcement official said.
Mohammad Jamil, a neighbor of Malik's family in her native Pakistan, said, "We felt very sad about what Tashfeen had done." He called her actions "shameful for us all."
Studies in Pakistan
Malik came from a family of wealthy landowners in Pakistan. She later moved to Saudi Arabia with her immediate family.
She returned to Pakistan to study pharmacy in the city of Multan between 2007 and 2012, according to Bahauddin Zakariya University, the school she attended.
One of Malik's professors, Dr. Syed Nisar Hussain, told CNN he didn't recall her being anymore religious than other students. Like most other young women at the school, she wore a veil over her face known as a niqab, he said.
"She always remained busy in studies," Hussain told CNN.
One of Malik's former classmates at the university also didn't remember her being very religious.
"She didn't even pray five times a day," the former classmate, who declined to be identified, told CNN. The two women both lived in the same off-campus housing, she said.
"She was a colorful girl who spoke about boys, social networks and chatting online," she said. "She wasn't some pure girl just because she took the veil."
Malik later attended the Al-Huda Institute in Multan, part of a chain of women-only religious schools in Pakistan. Al-Huda is known for its puritanical interpretation of Islam, but it isn't believed to be fundamentalist or to advocate violence.
In a statement, Al-Huda said Malik briefly studied at its Multan branch between 2013 and 2014 but left without completing the diploma course.
"It seems that she was unable to understand the beautiful message of the Quran," the organization said, stressing that it doesn't have any links to extremist groups and aims to promote a peaceful message of Islam.
'A stay-at-home mom'
Malik traveled to the United States in July 2014 on what's known as a "fiancee visa." Later she became a lawful permanent resident.
Mustafa Kuko, director of the Islamic Center of Riverside, said he counseled Farook as he was looking for a religious woman to marry. Farook, an American citizen who worked for the San Bernardino County health department, was a regular at the center.
Mohammad Abuershaid, an attorney for Farook's family, told CNN the couple met via an online dating website. They first met in person when Farook traveled to Saudi Arabia in 2013 during the Hajj pilgrimage, the attorney said.
"She was very conservative. She was a stay-at-home mom," Abuershaid said.
CNN has obtained a photocopy of Malik's Pakistani national identification card from a senior Pakistani intelligence source.
The card is issued to Pakistani citizens as proof of their citizenship and legal residence in the country.
This version suggests the card was issued before 2013, indicating Malik had been living in Pakistan during that time. The card shows her date of birth as July 13, 1986.
Couple had 6-month-old baby
Farook married Malik in a religious marital ceremony during a second trip to Saudi Arabia and then returned to the United States with his new wife. Kuko officiated the couple's U.S. marital ceremony at his mosque in 2014.
The couple had a 6-month-old daughter, whom they left with a grandmother the day of the shooting.
Malik and Farook lived in a rented two-bedroom, two-story townhouse
with their baby and Farook's mother, according to their landlord, Doyle Miller, the owner of the townhouse in Redlands, California, near San Bernardino.
He said the couple answered an ad on Craigslist.
No hints of extremism, family attorney says
"There was nothing to show that she was extreme at all," Abuershaid told CNN. "She was a practicing Muslim, she believed in the religion and she was just doing the five prayers a day, the fasting ...," he said.
There was nothing to show she was responsible for radicalizing her husband, he added.
Back in Malik's former neighborhood in Karor Lal Esan, Pakistan, Jamil said, "We don't want Muslims to do such things."
He added, "What Tashfeen did is not good for our country. There should be military courts for such people in Pakistan so that nobody dares to do such a bad thing in the future."
Malik's uncle still lives in the Pakistani town, according to Jamil. "He is a nice religious singer known as Naat Khawan," he said. "He is well-known by all people in the area."