Larry Klimek had just arrived at his mother's White Bear Township home for the annual New Year's potluck with his two kids. As he removed his shoes, Klimek's older brother suddenly appeared from the basement and shot him twice in the abdomen, a witness told investigators.

Klimek's 16-year-old son and 14-year-old daughter ran to a neighbor's to call 911.

Court filings unveiled those new details of last week's murder-suicide, but authorities say they still don't know what motivated 56-year-old Lee Klimek to fatally shoot his brother before killing himself.

"It's not real cut and dry on what prompted it," Sheriff's Office spokesman Randy Gustafson said Tuesday. "Did these brothers have a recent conversation … or was there something to trigger that kind of response?"

Around 20 relatives had gathered at the family matriarch's house to celebrate the holiday when the incident unfolded.

A Ramsey County sheriff's SWAT team responded to a call of shots fired inside the home on the 5400 block of Centerville Road at 3:45 p.m., and found people fleeing the premises. One witness told police that Lee Klimek was armed with a shotgun, rifle and handgun, according to dispatch audio.

The SWAT team cordoned off the block and tried to communicate by phone and loudspeaker with the gunman. Officers then deployed mobile cameras in the house. After two hours with no response, officers swept the property and found the brothers dead inside.

A cousin told police that Larry Klimek, a train and bus operator at Metro Transit, had just arrived with a cooler full of food, according to search warrants filed last week in Ramsey County District Court.

The party's host, 76-year-old Jeanette Klimek, reported that Lee Klimek lived in her basement and was aware of her party plans. She told investigators that she didn't know about any mental health problems with her son, but said he's been "in poor physical health and hasn't worked in 20 years," according to court records.

A search of the residence located the shotgun used in the shooting, as well as ample ammunition and a cellphone. A Gerber knife was also found in Lee's waistband, records show.

"Fortunately, this didn't turn into multiple victims," Gustafson said Tuesday. "Thank God that didn't happen."

Correction: Previous versions of this article included a headline that misstated Larry Klimek's city of residence. He lived in Minneapolis.