They knew the house far too well. It was where Christian Philip Oberender, then 14 years old, had murdered his mother in a shotgun ambush in the family rec room in 1995.

Now, 18 years later, Carver County Sheriff Jim Olson was sending his deputies back to the home where Oberender still lives. Just two days earlier, Olson had scanned the day's shift reports and froze when he tripped over Oberender's name. A scan of a Facebook page then showed firearms spread out like a child's trophies on a bed inside the home, along with notes about the Newtown, Conn., gunman who shot 20 children to death.

What Olson's deputies found in the home in Watertown Township was chilling: 13 guns, including semi-automatic rifles, an AK-47, a Tommy gun, assorted shotguns and handguns, including a .50-caliber Desert Eagle.

Even more disturbing was the letter Oberender had written recently to his late mother, Mary: "I am so homicide,'' it said in broken sentences. "I think about killing all the time. The monster want out. He only been out one time and someone die.''

Today, Oberender sits in a Carver County jail cell on a charge of being a felon in possession of firearms. And Olson, who investigated the 1995 murder as a young detective, finds his investigators at the center of a case that exposes the dangerous loopholes in the nation's gun laws and Minnesota's system of criminal background checks.

Even though Oberender killed his mother with a firearm, even though he was committed to the state hospital in St. Peter as mentally ill and dangerous more than a decade ago, he was able to obtain a permit to purchase firearms last May. That piece of paper gave Oberender, now 32, the ability to walk into any licensed Minnesota retailer and buy any assault weapon or pistol on the rack.

Dozens of other Minnesotans judged by a court to be mentally ill have also found that designation no barrier to obtaining deadly weapons.

A Star Tribune review of state court records found case after case in which individuals deemed mentally ill in judicial proceedings later wound up in possession of guns and accused of violent crimes.

At least 84 people have been charged since 2000 with illegal gun possession or assault with a dangerous weapon even though they had previously been committed by a judge as mentally ill. Of that group, 29 were charged with multiple counts of weapons possession and nine were considered by a judge to be mentally ill and dangerous.

Additionally, the Minnesota Bureau of Criminal Apprehension (BCA) has more than 168,000 "suspense files'' -- records on Minnesotans who have been arrested since 1990 but whose files are so incomplete that the state can't determine if they should have the right to buy guns.

"The system failed in this case,'' Olson said in an interview. "We are having discussions with the BCA to make sure there aren't similar things like this hanging out there.''

Federal agents from the Bureau of Alcohol, Tobacco and Firearms are working with Olson's investigators to sort out which guns Oberender might have bought from retailers using a permit he should never have possessed.

No red flags

How did Christian Oberender succeed in obtaining a gun permit?

The answer lies in a combination of deceit on his part, failures in the state court system, and haphazard data collection by state agencies, according to interviews with law enforcement officials.

In Minnesota, a person seeking a permit to purchase an assault weapon or pistol must submit an application to the local police or sheriff's department. There, the background check process begins with a query of the BCA's system. If no disqualifications show up -- such as a violent criminal record or mental illness commitment -- the permit is granted.

No state permit is required to purchase a long rifle or a shotgun in Minnesota. Buyers going to a licensed retailer must pass a federal background check at the counter -- but those records can also be incomplete because they are supplied to the FBI by state agencies.

Minnesota's gun laws don't require an applicant to provide a fingerprint or a Social Security number to verify identity.

"This was one of our concerns during the 'Conceal and Carry' debate in Legislature 10 years ago and it was beaten down like everything else," said Heather Martens, executive director of Protect Minnesota, a gun violence prevention organization.

Martens said Oberender's case highlights the reluctance of lawmakers to tighten gun laws because they fear being accused of infringing on individual rights. "Public schoolteachers have to go through a complete background check, even including a fingerprint,'' Martens said. "For buyers of assault weapons and pistols, law enforcement currently has only seven days to verify the person's identity and criminal history -- otherwise, a permit is automatically granted. We should at least allow police enough time to verify the person's identity.''