A married couple who killed 14 people in a California shooting rampage the FBI is investigating as an act of terrorism borrowed about $28,000 from an online lender, a sum deposited into their bank account about two weeks before the attack, a source said on Tuesday.

Disclosure of the unsecured loan the husband, Syed Rizwan Farook, 28, took out from San Francisco-based Prosper, a peer-to-peer lending service, offered a new glimpse into the money trail under scrutiny by investigators of last week's mass shooting.

The Federal Bureau of Investigation has described Farook, the U.S.-born son of Pakistani immigrants, and his Pakistani-born wife, Tashfeen Malik, 29, as a couple "radicalised" by Islamic extremist ideology.

Malik, who spent a good portion of her life in Saudi Arabia and married Farook there before returning with him to California in the summer of 2014, is believed by investigators to have pledged allegiance on Facebook to the leader of the militant group Islamic State just before the killings.

Authorities say the heavily armed couple opened fire on Farook's co-workers from the county Environmental Health Department during a holiday party at the Inland Regional Center social services agency in San Bernardino, about 60 miles (100 km) east of Los Angeles.

Fourteen people were killed and 21 others were wounded in the assault. The couple died several hours later in a shootout with police.

If the massacre - the deadliest burst of U.S. gun violence in three years - proves to have been the work of killers inspired by Islamic militants, it would mark the most lethal such attack in the United States since Sept. 11, 2001.

While investigations into assaults branded as acts of terrorism often focus on the money behind them, U.S. government officials said the FBI's examination of the couple's finances has not linked them with any foreign group.

Draining their accounts?

Still, one government source told Reuters that Farook and Malik apparently followed a pattern set by other militants who drained their bank accounts and exhausted credit lines before embarking on what they believed would be a suicide mission.

A separate source told Reuters that Prosper, a San Francisco-based online lender, made a $28,500 collateral-free loan to Farook in mid-November. Loans made by Prosper, which processes borrowers' applications and evaluates their credit-worthiness, are originated by the third-party bank WebBank, based in Salt Lake City. Prosper then sells its loans to investors.

Fox News first reported on Monday that a deposit of $28,500 was made into Farook's bank account from WebBank.com on Nov. 18, and that Farook converted $10,000 in cash, which he withdrew from a Union Bank branch in San Bernardino around Nov. 20. Fox also reported at least three $5,000 transfers were made in the days before the shooting, apparently to Farook's mother.

WebBank issued a statement expressing condolences to victims of the San Bernardino shooting but declined further comment, citing confidentiality restrictions.

In addition to the pair of assault-style rifles and semi-automatic handguns they carried the day of the killings, the couple were found to have amassed thousands of rounds of ammunition, along with explosives and other materials for making as many as 19 pipe bombs, according to the FBI.

One booby-trap consisting of three pipe bombs rigged to a remote-controlled device that failed to detonate was left by the killers at the scene of the attack, apparently intended to go off as police and emergency personnel swarmed the location, law enforcement officials said.

Probing gun connection

Enrique Marquez, a onetime friend of Farook who U.S. officials said had purchased the two assault rifles used in the attack, was questioned by investigators on Tuesday, according to a federal law enforcement official.

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.

While Farook and Malik may have been inspired by Islamic State or other extremists, U.S. government sources have said there was no evidence the San Bernardino attack was directed by the militant group or that Islamic State even knew of them.

FBI Director James Comey said on Friday that no information had been unearthed suggesting the killers were part of an extremist cell or network.

Still, issues of gun control, national security and immigration have reverberated in the U.S. presidential campaign from the first moment that authorities suggested that Islamic extremism may have played some role in last week's attack.

On Sunday President Barack Obama urged Americans in a nationally televised address to avoid the scapegoating of Muslims. But a day later, Republican presidential front-runner Donald Trump singled out Islam as a threat by calling for a blanket prohibition 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."

Even before the San Bernardino killings, the Obama administration had drawn fire from political conservatives for its plan to allow 10,000 refugees from Syria's civil war into the United States, feeding concerns about anti-Islam sentiments.

Police in Philadelphia were investigating an incident in which a pig's head was thrown at a mosque early on Monday morning.