The FBI says it is investigating the deadly mass shooting in California as an "act of terrorism".
The husband and wife shooters attempted to destroy evidence, including crushing two mobile phones and discarding them in a rubbish bin, said David Bowdich, assistant director of the FBI's Los Angeles office.
He said authorities were continuing to investigate the case to understand the motivations of the pairs and whether they were planning more attacks.
Fourteen people died in the San Bernardino massacre.
Malik and Syed Farook targeted people at a holiday party for his co-workers. The Muslim couple were killed hours later in a gunbattle with police.
Malik, 27, was a Pakistani who grew up in Saudi Arabia and came to the US in 2014 on a fiancee visa. Farook, a 28-year-old restaurant health inspector, was born in Chicago to Pakistani parents and raised in California.
A law enforcement official said Malik pledged allegiance to IS and its leader on Facebook, making her posts under an alias.
Facebook discovered the account on Thursday and removed the profile from public view.
Another US official said Malik expressed "admiration" for the extremist group's leader on Facebook under the alias account. But there was no sign that anyone affiliated with the Islamic State communicated back with her.
The two officials asked not to be named.
Separately, a US intelligence official said that Farook had been in contact with known Islamic extremists on social media.
Law enforcement officials have long warned that Americans acting in sympathy with Islamic extremists - though not on direct orders - could launch an attack inside the US. Using slick propaganda, the Islamic State in particular has urged sympathisers worldwide to commit violence in their countries.
Two weeks ago, with Americans on edge over the Islamic State attacks in Paris that left 130 people dead, FBI Director James Comey said that authorities had no specific or credible intelligence pointing to an attack on American soil.
Seventy-one people have been charged in the US since March 2014 in connection with supporting ISIS, including 56 this year, according to a recent report from the George Washington University Programme on Extremism. Though most are men, "women are taking an increasingly prominent role in the jihadist world," the report said.
It was not immediately clear whether Malik exhibited any support for radical Islamists before she arrived in the US - or, like scores of others arrested by the FBI, became radicalised online or by meeting someone.
To receive her visa, Malik was subjected to a vetting process. It includes in-person interviews, fingerprints, checks against terrorist watch lists and reviews of her family members, travel history and places where she lived and worked.
Foreigners applying from countries that are home to Islamic extremists - such as Pakistan - undergo additional scrutiny before the State Department and Homeland Security approve their applications.
Pakistani intelligence officials said Malik moved as a child with her family to Saudi Arabia 25 years ago.
They said that the family is originally from a town in Punjab province and that the father initially moved to Saudi Arabia around three decades ago for work.
Farook had no criminal record and was not under scrutiny by local or federal law enforcement before the attack.
"This was a person who was successful, who had a good job, a good income, a wife and a family. What was he missing in his life?" asked Nizaam Ali, who worshipped with Farook at a mosque in San Bernardino.
Authorities said that the couple sprayed as many as 75 rounds into the room before fleeing and had more than 1,600 rounds left when they were killed. At home, they had 12 pipe bombs, tools to make more explosives and well over 4,500 rounds, police said.
The dead ranged in age from 26 to 60. Among the 21 injured were two police officers hurt during the manhunt. Two of the wounded remained in critical condition.
The soft-spoken Farook was known to pray every day at San Bernardino's Dar Al Uloom Al Islamiyah mosque.
The last time a friend, Rahemaan Ali, saw him was three weeks ago, when Farook stopped coming to pray. Rahemaan Ali said Farook seemed happy and his usual self.