Malik's relatives in Pakistan have said she appeared to have abandoned the family's moderate Islam and become more radicalized during years they spent in Saudi Arabia.
Malik, 29, and her US-born husband, Syed Rizwan Farook, 28, stormed a gathering of civil servants in San Bernardino, California, on Wednesday, opening fire with assault rifles and killing 14 people.
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The pair were killed two hours later in a shootout with police SWAT team members. "There's a serious investigation ongoing into what she was doing in Pakistan and in Saudi," McCaul said on Fox News Sunday.
"We think that she had a lot to do with the radicalization process and perhaps with Farook's radicalization from within in the United States."
"The wild card here is the wife Malik," said McCaul, who said that investigators were also investigating where they got the money to acquire the guns.
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Asked what ties the couple had with the Islamic State, which has said the pair were "followers", McCaul said that was unclear, and was a part of the investigation that was underway.
At a minimum, he said, Islamic State "inspired" the attack since Malik pledged allegiance to the group in a Facebook posting. "We have the computers. We have the devices. We are currently going through the forensics," he said.
"The investigation is ongoing to find out what is precisely the connection between ISIS, say, in Raqqa and in the United States, and what was going on."
US officials have acknowledged that they had no information about the couple before the killing other than a routine request by Farook to grant Malik a visa so she could join him in the United States so they could marry.
McCaul also noted that Farook had a large arsenal of semi-automatic guns, hundreds of rounds of ammunition and pipe bombs. "We are looking at the terrorist financing aspect to this case. I believe on his salary, he was not able to buy this on his own," said McCaul.
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