RIVERSIDE (CBSLA.com) — The man authorities believe bought assault weapons used in the San Bernardino mass killing has checked into a psychiatric hospital, CBS News has confirmed.

David Begnaud, a freelance correspondent for network, reported the news Sunday via Twitter.

The mass killing at the Inland Regional Center on Dec. 2 claimed 14 lives and left 21 others injured. The suspected killers, Syed Farook and Tashfeen Malik, were killed in a gun battle with police shortly after the rampage.

While federal investigators continue to determine a motive for the killings, the Pakistani-born Malik, who entered the U.S. on a K-1 visa, is believed to have pledged allegiance to the Islamic State group in a Facebook post around the same time the attack was launched, according to the Associated Press.

In the final years of her life, the AP reports that Malik urged people to live a more devout life.

COMPLETE COVERAGE: San Bernardino Shooting

Overnight Saturday, an army of federal agents served search warrants at a Riverside County home next to the home where Farook once lived.

The FBI has not disclosed what, if anything, the agency found at the home, nor what they were seeking. The warrant was sealed.

The FBI also had not disclosed whether anyone was detained or arrested, although CBS News reports that the neighbor was not placed under arrest nor considered a suspect.

Three agents removed a box from the home on Sunday morning.

A law enforcement source told CBS News that they’re confident that no one aside from Farook and allegedly the neighbor, identified as Enrique Marquez, were involved in the purchase of the guns.

According to the network, the source says law enforcement believes Farook and Marquez allegedly reconfigured the guns to be more powerful after buying them legally in California.

Growing up, neighbors say Farook and Marquez were “always together” usually working on Farook’s car.

“They were very quiet and then they found each other and you know they were best buddies for a long time,” said a neighbor.

The network also reports that law enforcement officials are reviewing video from gun-store owners who called to report they saw the pair together.

According to Ed Mertz of KNX 1070 NEWSRADIO, the FBI says they want to question the man, but he checked himself into a psychiatric hospital after the attack.

According to CBS News, authorities are expected to defer to the U.S. Attorney’s office for the next steps.

Meanwhile, Farook’s father says his son was fixated with on Israel and supported the Islamic State, according to an Italian newspaper report.

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