RIVERSIDE: The man who supplied guns to the couple who killed 14 people in San Bernardino, California, last week is connected to the shooters' family by marriage, state documents show.

Enrique Marquez, whose home was raided over the weekend and who is being questioned by federal investigators, was married last year to Mariya Chernykh, whose sister is the wife of Raheel Farook, brother of one of the shooters, Syed Rizwan Farook.

The family connection, revealed in marriage documents seen by Reuters, adds another element to the relationship between Farook and Marquez.

Read: The brothers Farook: one a decorated US Navy veteran, the other a gunman

Two assault-style rifles used in the attack were legally purchased by Marquez, who has been described by several of his neighbours as having been a good friend of Farook, according to the US Bureau of Alcohol, Tobacco, Firearms and Explosives.

The New York Post reports that Farook's father has been placed on an FBI watch list due to his son's activities and his own personal links to Pakistan.

The Farook brothers lived next door to Marquez when they were growing up, though the brothers had both since moved.

Raheel Farook, the older brother and who became the brother-in-law of Marquez, served in the US Navy, joining in 2003 shortly after graduating from high school.

Federal agents raided the Marquez home in Riverside, California, on Saturday.

Marquez was being interviewed on Tuesday, a law enforcement official said. He had checked himself into a Los Angeles-area psychiatric facility following the shooting.

The marriage documents show that Enrique Marquez married Mariya Chernykh on November 29, 2014.

It could not be determined whether they currently live together. Witnesses to the marriage were Mariya's sister, Tatiana Farook, and Tatiana's husband, Raheel Farook.

Raheel Farook married Tatiana, then known as Tatiana Gigliotti, on September 10, 2011, the documents show.

Two neighbours of Marquez told Reuters they never saw him with a woman.

"I saw no sign of him having a wife - it was only his mom in the house," said Lori Aguirre, who lives across the street and said she knew Marquez because he sometimes drove her son to and from school.

"I only saw him going to work or coming home alone. It would be a big surprise if he was married," she said.

This was echoed by another neighbour, who said he did not know Marquez.

"I never saw any young woman around. Only his mom," said the man, who declined to be named.

A third neighbour, however, said he had seen Marquez with a woman. "I don't know if they were legally married, but I saw him with someone," said the neighbour, who also declined to be named.

Employees at a Walmart Supercentre in Corona, California, 20 minutes by car from Riverside, said Marquez has been working there as an asset protection and customer specialist for about six months. He was last seen at the store on November 29, they said.

Ashlee Sims, 25, a security guard at the Walmart, on Sunday described Marquez as "weird" and "awkward." He is quiet, but likes to go out and party, Sims said.

California shooter took out $28,500 loan: source

United States (US) investigators are looking into the California shooters' financial transactions leading up to the attack last week, including a $28,500 loan, a source close to the investigation said Tuesday.

Syed Farook took out the online loan through San Francisco-based Prosper Marketplace, the source told AFP.

He then made a withdrawal of $10,000 around November 20 in San Bernardino, the city of some 200,000 people where he and his wife carried out the deadly attack approximately two weeks later.

According to Fox News, investigators are trying to determine whether the $10,000 withdrawal from Union Bank was used to pay Enrique Marquez, a friend of Farook who bought the two AR-15 automatic weapons used in the bloodbath.

If so, Marquez could face prosecution, particularly if he made the illegal modifications found on the weapons, Fox said.

Additionally, in the days leading up to the massacre, there were at least three transfers of $5,000 into the account of Farook's mother, Rafia.

Joint pledge?

Authorities have said Farook, 28, and wife Tashfeen Malik, 29, were radicalised Muslims who committed an "act of terrorism" when they massacred 14 people on December 2.

Farook, the United States (US)-born son of Pakistani immigrants, and Malik, who was born in Pakistan and spent much of her life in Saudi Arabia, died in a shootout with police several hours after the attack in San Bernardino, about 100 km east of Los Angeles.

Read: Suspects Syed Farook, Tashfeen Malik kill 14 in California shooting: authorities

US investigators say they are examining a Facebook posting in which Malik is believed to have pledged allegiance to IS chief al-Baghdadi, made around the time of the attack.

However CNN, citing a US official, said that the final Facebook post used the word "we," indicating that the declaration of allegiance came from both Farook and Malik.

US-born Farook was a health inspector for San Bernardino County earning $53,000 a year, according to Fox News.

Farook had good credit that facilitated his loan, the source close to the case told AFP.

Meanwhile, the Utah-based online bank that financed the loan, WebBank.com, said it was prohibited from "commenting on any specific loan borrowers or applicants."

"WebBank evaluates all loan applications in accordance with legal requirements, including US anti-terrorism and anti-money-laundering laws," it added.

Prosper Marketplace also said its loans were "subject to all identity verification and screening procedures required by law."

The New York Post reported that Farook's father has been placed on an FBI watch list due to his son's activities and his own personal links to Pakistan.

According to the paper, Farook's father, who shares the same name as his son, arrived in the US in 1972 and obtained US citizenship in 1999.