Kevin Janson Neal made two of the rifles — AR-15 semi-automatics — he used in California's mass shooting on Tuesday, getting around the state’s strict gun laws, a sheriff's officer and experts said.

Neal, 44, reportedly made the rifles from parts he had purchased, Phil Johnston, Tehama County assistant sheriff, told the Redding Record Searchlight on Wednesday.

Officials also said two other rifles in Neal's possession were not registered to him, but someone else, the Record Searchlight reported.

Neal's rampage left six dead and 10 injured, the newspaper said, and Neal was killed during a shootout with deputies as he tried to escape in a stolen vehicle.

"These firearms were manufactured illegally, we believe, by him at his home," Johnston said, per Record Searchlight. "So they (the guns) were obtained in an illegal manner, not through a legal process. They are not registered."

NBC News reported experts saying that Neal appeared to have used a legal loophole that allowed him to get around California's gun laws by ordering the parts for a weapon and putting it together himself.

"As long as it's not restricted in the state of their residence and the person is not prohibited, anyone can manufacture a sporting firearm for personal use," Rick Vasquez, a retired technical expert with the federal Bureau of Alcohol, Tobacco, Firearms and Explosives, told NBC News.

"These firearms are not required to have a serial number."

Vasquez added that most people who put together their own guns legally are hobbyists.

"But as with any type of device, they also can be used in a criminal activity by bad actors who are looking for ways to acquire firearms without any traceability," Vasquez told NBC News. "It's past the point of no return."

Bob Thacker, president of the Pennsylvania Gunsmith School, told NBC News that guns put together from kits can skirt most gun control laws because the unassembled pieces are not viewed as guns, but just a collection of parts.

"Any firearm that is transferred by a dealer to an individual goes through background check,” Thacker said, per NBC News. "Firearms are mechanical. Pieces and parts go together ... Anybody can learn to do it."