US investigators are increasingly convinced the California shooters planned multiple attacks, given their stockpile of weapons, and are looking at whether the Pakistani woman involved radicalised her American husband.
Investigators believe the weapons cache collected by Tashfeen Malik, 29, and her husband, Syed Rizwan Farook, 28, points to more attacks but they do not have evidence on other possible targets, a senior US government source told Reuters on Sunday.
The couple stormed a gathering of his work colleagues in San Bernardino, California, on Wednesday, opening fire with assault-style rifles and killing 14 people. The pair were killed a few hours later in a shootout with police.
US authorities are trying to learn what contacts Malik might have had with Islamic militants in Pakistan and Saudi Arabia, where she grew up, the official said on condition of anonymity.
They lack clear evidence that the wife was radicalised overseas or that she in turn radicalised her husband, though they are actively investigating that, the official said.
Authorities are investigating the shootings as an act of terrorism.
Malik's estranged relatives in Pakistan have said she appeared to have abandoned the family's moderate Islam and become more radicalised in Saudi Arabia, where she moved as a toddler.
She returned to Pakistan and studied pharmacy at Bahauddin Zakaria University in Multan from 2007 to 2012.
"There's a serious investigation ongoing into what she was doing in Pakistan and in Saudi," US politician Michael McCaul said on Fox News Sunday.
"We think that she had a lot to do with the radicalisation process and perhaps with Mr Farook's radicalisation from within the United States.
"The wild card here is the wife Malik," said McCaul, the Republican chairman of the House homeland security committee. He said investigators were also looking at where they got the money to acquire the guns.
US Attorney General Loretta Lynch said US authorities have no evidence the shooters were part of a larger terrorism cell, but were working with their counterparts overseas to gather information about their lives.
"We are trying to learn everything we can about both of these individuals," Lynch said on NBC's Meet the Press. "It will be a long process....
"And we are trying to learn as much as we can about her life before they met, after they met and frankly, after she came here as well. What we are trying to focus on again is what motivated these two individuals."
McCaul said it was unclear what ties the couple had with the Islamic State, which has said the pair were "followers". At a minimum, he said, the militant group inspired the attack. Malik is believed to have pledged allegiance to the group in a Facebook posting shortly before the shooting.
"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."
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.
US officials have acknowledged they had no information about the couple before the killing other than routine matters related to Malik's immigration status in the US.
Susan Rice, Obama's national security adviser, said the case illustrates the difficulties in detecting self-radicalised attackers.
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