Tashfeen Malik (left) and Syed Farook are pictured passing through Chicago's O'Hare International Airport, July 27, 2014. Reuters/US Customs and Border Protection/Handout via Reuters

By Mark Hosenball and Michael Erman

WASHINGTON/NEW YORK (Reuters) -- Syed Rizwan Farook recently took out a $28,500 loan from an online lender, a source said on Tuesday, before he and his wife killed 14 of his co-workers at a holiday party in San Bernardino, California.

Authorities have said Farook, 28, and wife Tashfeen Malik, 29, were radicalized Muslims. The FBI has said that the December 2 attack is being investigated as an "act of terrorism."

While investigations into such attacks often focus on how they were financed, U.S. government officials said the FBI's examination of the couple's finances has not linked them with any foreign group.

A source told Reuters that Prosper, a San Francisco online lender, made the $28,500 loan to Farook. Prosper evaluates borrowers for loans, which are originated by third-party bank WebBank. Prosper then sells the loans to investors.

One of the Reuters government sources said Farook and Malik apparently pursued a scenario previously followed by U.S.-based militants by draining their bank accounts and taking credit lines to their maximums before embarking on what they believe was a suicide mission, knowing that they would not have to repay the debts.

Fox News first reported on Monday that Farook engaged in transactions involving more than $28,000 and that the money originated with WebBank.com around Nov. 18. It said investigators were trying to determine if the money was a loan taken out by Farook.

WebBank, based in Salt Lake City, did not immediately respond to a request for comment.

Farook, the U.S.-born son of Pakistani immigrants, and Malik, who was born in Pakistan and spent most of her life in Saudi Arabia, died in a shootout with police several hours after their attack last Wednesday morning at the Inland Regional Center social services agency in San Bernardino, about 60 miles (100 km) east of Los Angeles.

Malik was believed to have pledged allegiance on Facebook book to the leader of the militant group Islamic State around the time of the shooting.

A federal law enforcement official said Enrique Marquez, who has been described as a close friend of Farook, was being interviewed on Tuesday about the weapons used in the attack, including two assault-style rifles he had legally purchased.

Marquez' home in Riverside was raided by federal agents over the weekend. Federal law enforcement sources told Reuters that Marquez had checked himself into a Los Angeles-area psychiatric facility soon after the shooting.

If the shooting proves to have been the work of people inspired by Islamic militants, it would mark the most lethal such attack in the United States since those by al Qaeda on Sept. 11, 2001.

The San Bernardino killings have made Muslim in America an issue in the U.S. presidential race. On Sunday President Barack Obama urged Americans not to make Muslims scapegoats for the attack but a day later, Republican presidential front-runner Donald Trump called for a ban on Muslims entering the United States.

"We have no choice but to do this," Trump told ABC News on Tuesday. He said the ban would be temporary. "We have people that want to blow up our buildings, our cities. We have to figure out what's going on."

At a meeting of county supervisors in neighboring Riverside County on Tuesday, officials paid tribute to the victims, including one who worked for Riverside County.

(Additional reporting by Julia Edwards in Washington; Writing by Bill Trott; Editing by Jonathan Oatis, Toni Reinhold)