SAN BERNARDINO, Calif. (Reuters) - 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, sources said on Tuesday.

Tashfeen Malik, (L), and Syed Farook are pictured passing through Chicago's O'Hare International Airport in this July 27, 2014 handout photo obtained by Reuters December 8, 2015. REUTERS/US Customs and Border Protection/Handout via Reuters

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 “radicalized” 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.

The wife’s extremist views took form before she came to the United States, but it remains to be seen whether she and her husband were indoctrinated by other individuals or whether they turned to radical ideology on their own, the FBI has said.

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.

In yet another twist to the case, state marital records examined by Reuters on Tuesday revealed that the man said by authorities to have furnished the couple with the two assault-style rifles used in the attack was related by marriage to Farook’s family.

Specifically, Enrique Marquez, whose home was raided over the weekend and was being questioned by federal investigators on Tuesday, became married last year to Mariya Chernykh, whose sister is married to Farook’s older brother, Raheel Farook.

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 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.

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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

Meanwhile, the family connection revealed between Marquez and Farook added a new dimension to links under investigation between the two men.

Neighbors of Marquez have said the two had been close friends, and the Farook brothers lived next door to Marquez when they were growing up, through the brothers had since moved.

Federal law enforcement sources told Reuters that Marquez, who they characterized as a key witness, had checked himself into a Los Angeles-area psychiatric facility soon after the shooting, but they were interviewing him on Tuesday.

Officials for the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives have previously said all the guns seized in the investigation had been purchased legally from licensed firearms dealers in California between 2007 and 2012, with Marquez identified as the buyer of the two assault rifles.

The marriage documents show Marquez and Chernykh were wed on Nov. 29, 2014, a few months after Farook and Malik were married and returned to the United States. Raheel Farook and his wife, Tatiana, were witnesses to the Marquez-Chernykh nuptials.

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.