A 20-year-old man from Staples, Minn., is accused of causing catastrophic brain injuries to a 3-month-old infant in his care last month who later died.

Presley A. Sleyster was charged Friday in Cass County District Court with second-degree unintentional murder in connection with the death of his wife’s infant son.

Sleyster told authorities he was not the biological father but thought of the child as his own.

According to the criminal complaint:

Sleyster, who is unemployed, moved in with his wife’s family in Staples just before their Nov. 15 wedding. He was acting as the child’s primary caregiver while she was at work.

On Dec. 5, Sleyster told authorities, he was alone with the child for the afternoon. When he woke the baby from a 4½-hour nap to feed him, the infant immediately started screaming, he said. This continued for about 10 minutes, he said, before the child suddenly went “stiff as a board,” according to court records.

Sleyster called a grandmother for help before dialing 911. He told emergency responders that the child had a seizure and was struggling to breathe.

The baby was flown to the St. Cloud Hospital and later transferred to Children’s Hospital in Minneapolis, where he died two days later.

When asked by police, Sleyster repeatedly denied that the baby had fallen, was dropped or struck. “We’re all puzzled,” he said.

According to the criminal complaint, a doctor determined that the child’s injuries “definitively” resulted from head trauma and “did not occur spontaneously or due to other medical illness.”

An autopsy by the Hennepin County Medical Examiner’s office later concluded that the baby died of “blunt force homicidal violence.”

A week after the incident, Sleyster told police that he may have accidentally hit the child’s head on a door frame or railing while running upstairs with him.

He also told investigators that it could have happened while cradling the child in bed. Sleyster said he turned quickly to reach for the bottle and may have hit the baby’s head on the nightstand.

Sleyster said he’d told his wife that he accepted responsibility for what happened.

“I’m the one at fault for this [because] it happened in my care,” he said.