FBI agents early Saturday raided the California home of a man believed to have bought the assault rifles used in the San Bernardino shooting rampage that killed 14 people, according to published reports.

As the FBI investigates the attack as an act of terrorism possibly inspired by ISIS, the agency is now working to determine if the man, a former neighbor of one of the attackers -- Syed Rizwan Farook -- had any knowledge of the plot, The Washington Post reports.

The search was conducted at the Riverside home of Enrique Marquez, the paper reported Saturday night. Farook lived in the home next door with relatives until a few months ago.

Agents removed bags and boxes of evidence from the home, Fox News reported Sunday.

Marquez is thought to have bought the two military-grade rifles used in the deadly attack, according to the Post. The weapons had been modified to make them more lethal.

Marquez, who has not been charged with a crime, has checked himself into a nearby mental health facility, the paper reported.

The Los Angeles Times, citing a source, reported Sunday that the FBI is seeking to interview Marquez about the rifles.

The paper reported that the guns were purchased three years ago.

Neighbors told the paper that while Farook was a loner, the only exception was his friendship with Marquez. They apparently bonded over a shared passion in tinkering with cars, the Times reported.

On Thursday, Gustavo Ramirez, who said he was Marquez’s stepfather, told a Times reporter that he and his wife hadn’t heard from Marquez since Wednesday, the day of the shooting, and were concerned. Ramirez said it was unlike Marquez not to come home.

On Friday, someone put up a sign in the front yard of the Marquez home that said: “Please keep off the property thank you.”

During the search of the home, neighbors told NBC News, FBI agents cut open the garage door with blow torches and used bomb-sniffing dogs during the search.

NBC reported that Marquez’s younger brother and his father were detained temporarily.

The father told NBC Saturday afternoon that he does not know where his son is and declined further comment.

Wearing tactical gear, Farook and his wife Tashfeen Malik stormed a holiday party December 2 at San Bernardino’s Inland Regional Center for Farook’s co-workers and fatally shot 14 people and injured 21 others. They were killed in a furious gun battle with police several hours later near their home in a California suburb.

ISIS says the couple were its followers and a source told Reuters Sunday that investigators are increasingly convinced they may have been planning multiple attacks given the weapons cache in their possession.

The FBI said Farook legally bought the two handguns used in the attack -- purchases that would have required a background check. And there is no indication he or Malik had any criminal record or history of mental illness that would have triggered California's unique law allowing authorities to seize weapons from those who aren't allowed to own them.

Federal officials are investigating whether the military-style rifles used were part of an illegal straw purchase, and then given to Farook or Malik, the Associated Press reported Sunday.

It was not until Friday that the FBI said it was investigating the massacre as a terrorist attack. If that's proven, it would be the deadliest strike by Islamic extremists on American soil since Sept. 11, 2001.

Farook, born in Chicago to Pakistani parents and raised in Southern California, met his wife online. She was from Pakistan but traveled to Saudi Arabia where her family lived. Malik arrived in the U.S. on a K-1 visa for fiancees in July 2014, and the couple married the following month in Riverside. They had a 6-month-old daughter.

He attended a mosque daily in Riverside until two years ago, when he began going to another mosque in San Bernardino. He was a daily attendee there as well, but suddenly stopped going three weeks before the shooting, according to brothers Nizaam Ali and Rahemaan Ali, who attend the mosque.

Very little is known about Malik. She studied pharmacy at Bahauddin Zakariya University in the Pakistani city of Multan in 2012, according to the university's vice chancellor. A maid who worked in the Multan home where Malik lived said she would travel back to Saudi Arabia to be with her family when school was out. During her time in Multan, her style of dress became more conservative over time.

Farook family attorneys have described her as "just as a housewife" who was soft-spoken and little known, even to the family and the mother-in-law who lived with her. Following religious tradition in their home, men and women would remain separated during social visits.

A U.S. law enforcement official told The Associated Press that Malik used an alias on Facebook to make a declaration of support for the Islamic State and its leader. But there is no sign anyone from the group communicated with her or provided any guidance for the attack. A Facebook official told AP the post went up at 11 a.m., around the time the couple began their attack.

David Bowdich, assistant director of the FBI's Los Angeles office, said investigators determined the shooters attempted to destroy evidence, including crushing two cellphones and discarding them in a trash can. Authorities are trying to retrieve the data.

FBI Director James Comey noted the bureau's investigation so far has shown no evidence that the shooters were part of a larger group or members of a terror cell.

The Associated Press contributed to this report.