In the late summer of 1981, LaDonna Mathis, a 30-year-old mother, was living with her family on their farm north of Mount Vernon, South Dakota. After a fire had destroyed their house, they had converted a metal machine shed into temporary living quarters. She and her husband, John, had three sons: four-year-old Brian, two-year-old Patrick, and an infant son, Duane. It was a makeshift home, but it was theirs as they worked to rebuild.
In the pre-dawn hours of September 8, 1981, a horrific scene shattered the quiet of the rural community. John Mathis contacted authorities to report a home invasion. When the first deputy, a high school classmate of John's, arrived at the farm, he found a chilling scene inside the crowded shed. LaDonna and her two young sons, Brian and Patrick, were dead in their beds, all having been shot. John Mathis had a gunshot wound to his left arm but was alive.
The couple's youngest son, a nine-month-old infant, was unharmed, as he had been staying the night with his maternal grandparents.
LaDonna had been shot twice in the head as she slept. Brian was shot once in the ear. Patrick was shot first in the back of the neck and then again in the eye, a detail that suggested he may have been kneeling in his bed. The weapon used was a.22-caliber semiautomatic rifle. John Mathis told investigators that a masked intruder was responsible for the carnage. He claimed he had taken his son Patrick to an outhouse around 2 a.m. and then went to check on a hog. He said he heard a car and saw lights on in the shed. A man in a dark stocking mask emerged, and during a struggle, John was shot in the arm and lost consciousness. When he awoke, the man was gone, and his family was dead.
Investigators from the Davison County Sheriff's Office and the South Dakota Division of Criminal Investigation (DCI) were immediately suspicious of John's account. As the investigation progressed, authorities felt that certain details were not adding up. Their suspicions soon centered on John Mathis as the sole suspect in the triple homicide. He was arrested and charged with three counts of murder.
The subsequent trial in 1982 became what the state's then-Attorney General, Mark Meierhenry, called the "most-covered, watched and listened-to case in the recent history of the state." The prosecution laid out its case, but after deliberation, a Yankton County jury found John Mathis not guilty of the murders of his wife and two sons. He was acquitted and walked free. No other suspects have ever been publicly identified, and no one else has ever been charged in connection with the crime.
For decades, the case has remained a painful, unresolved mystery in South Dakota. Some law enforcement officials involved in the original investigation have remained convinced of John Mathis's guilt. Former Davison County Sheriff Lyle Swenson, decades after the murders, recalled having a strange feeling from the moment he was notified, telling his wife, "That damn Mathis shot his family." The case has been the subject of books and podcasts, with author Noel Hamiel calling it "one of the most notorious murders in South Dakota history," noting the lack of any clear motive for an outside intruder.
As of October 2022, more than four decades after the tragedy, the Mathis family homicides remain an open and unresolved cold case. The South Dakota Division of Criminal Investigation continues to list the murders of LaDonna, Brian, and Patrick on its official list of unresolved cases. The rifle used in the killings was never recovered. The surviving son, who was with his grandparents that night, grew up and eventually came to live in the rebuilt house on the farm, next to the very site where his mother and brothers lost their lives.
