Sioux Falls Police are seeking answers about what may have caused a man to shoot his fiancee, then turn a gun on himself Wednesday in a murder-suicide at the couple's central Sioux Falls home.

A relative found John Ernest Vigus, 32, and Kayla Marie Paetow, 20, dead in their central Sioux Falls home after hearing gunshots just before 2 p.m. Wednesday.

A police spokesman said Vigus shot Paetow before turning the gun on himself, but the circumstances that led up to the shooting are still unclear.

A witness told investigators the couple had a "minor" argument prior to the shooting, Lt. Matt Burns said, but police don't know if it was a factor in the murder-suicide.

Vigus' 6-year-old son and Paetow's older brother were upstairs at the time. Paetow's brother called police at 1:54 p.m. after hearing gunshots and discovering the bodies on the first floor. The boy has been placed in protective custody with Social Services.

Vigus and Paetow were pronounced dead when police arrived, Burns said. Autopsies were pending.

"At this time our investigation is still going on as to what led up to this incident," Burns said.

A former co-worker, Sarah Katherine Smith said she worked with Vigus and Paetow at Freeman Regional Hospital. While the couple had their share of problems, she said she wouldn't have ever guessed something like this would have happened.

"Kayla was an exceptionally strong girl because of all the stuff she had gone through in the last couple of years. She was a little quieter, but a real sweetheart. Very level-headed and loved people a lot," Smith said. "John I knew the best and considered him one of my best friends. (I've) known him for about five years and was always crying from laughing so hard around him."

Paetow recently lost a child. Her two-month old daughter, Kaydence, died Jan. 10, 2013, as a result of SIDS, according to an obituary that ran in the Argus Leader.

Kayla's brother, Kyle Paetow, said his sister had potential to do whatever she wanted in life.

"You never know how much you truly love somebody until they're gone. Kayla and I have been really close ever since we were little," Kyle said. "I couldn't have asked for a better sister, she was always there for me whenever I needed her, no matter what."

Vigus also went through hard times, according to a friend, Zac Garber, but he had always pulled through. When Garber heard the news, he said it felt like "time stopped."

"Nothing can describe my feeling for his family and his son, who I got to know well, too," he wrote in a message to the Argus Leader.

Police said the couple moved to Sioux Falls from Freeman about a month ago. Investigators plan to speak with Freeman and Hutchinson County law enforcement to see if they have ever had any contact with the couple, Burns said.

Alcohol containers were found inside the residence, but it is unclear whether drinking was a factor, Burns said. He said police hadn't been called to the residence in the past month.

Burns acknowledged that without a witness to the shooting there is a chance the motive will remain unknown.

"The dynamic of the relationship and what occurred there could have been a heat of passion or a sudden provocation type thing," Burns said.

The murder-suicide is Sioux Falls' fourth homicide this year.