Police say they found Christian Oberender with 13 guns, despite the fact he couldn't legally purchase any.

A Minnesota man who killed his mother with a shotgun and who has a history of mental illness managed to amass a personal arsenal in recent years, according to court documents.

In early January, police arrived at the home of Christian Philip Oberender to find the 32 year old in possession of 13 guns, including an AK-47, shotguns, and a Tommy gun, according to a complaint filed in Carver County’s 1st Judicial District Court on January 9.

Police say they also found a note from Oberender addressed to his deceased mother in his house, according to the court document.

“I feel the good part of me fade away. I don’t know how long I can hold it in for,” the note read, according to the court document. “The monster want out. I know what happens when he comes out. He only been out one time and someone die.”

Oberender had been adjudicated a delinquent in 1996 for killing his mother, according to the document. He was civilly committed as a “mentally ill and dangerous person” in 1998.

The Carver County Sheriff became interested in Oberender after receiving a tip that he had posted Facebook pictures of himself toting assault weapons and expressed sympathy for the shooters at Columbine High School and in Newtown, Conn., according to the document.

For one community leader, the move by police to seize Oberender’s firearms came none too soon.

“The neighbors said they made numerous calls to the sheriff’s department that a young man is out shooting a gun in the back yard,” local school superintendent David Marlette told NBC affiliate KARE. “I just think it took too long for someone to come and take his guns away.”

Oberender was charged with being a felon in possession of firearms, and booked into Carver County jail. He remained there Monday night with bail set at $100,000, according to a Carver County Jail inmate roster.

According to the Minneapolis Star Tribune, Oberender obtained a permit in May that allowed him to buy guns through Minnesota dealers.

Custer County Deputy Jason Kamerud told KARE that Oberender might have been able to buy the guns himself. The convicted killer’s name did not show up in a background check through the state’s Bureau of Criminal Apprehension database, according to Kamerud.

“When we ran his criminal history, it wasn’t indicated that he should not be able to have firearms,” Kamerud said.

There was no record because the BCA was never given any information on Oberender, according to officials.

"The BCA relies on entities in the criminal justice system to provide data on an individual which then populates the individual's criminal history," BCA officials said in a statement, according to KARE-TV. "There were no data submitted to the BCA about this individual. Without it there can be no record."

Oberender lived in treatment centers until he was 21, according to a 2003 article by the Associated Press. He then spent a year in a halfway house before being released, according to the article. At the time of his interview with the AP, Oberender said he was working in an auto parts store.

“I saw all kinds of psychologists and got all kinds of treatment,” Oberender told the AP. They helped him “manage my behavior and not get angry over stupid stuff,” Oberender said at the time.