A 34-year-old St. Paul man was charged Thursday with second-degree murder in the June death of his girlfriend's 3-year-old son.

Devioun Keith Johnson was caring for the child and his older brother while their mother was at work when the younger boy stopped breathing, according to the complaint released by the Ramsey County attorney's office.

The child's mother told police that she left her sons with Johnson at 5 p.m. June 24 at her apartment in the 1200 block of Herbert Street and that she returned at 12:30 a.m. on June 25, the complaint said.

"The children were fine when she left. [They] were already in bed when she arrived home and she did not check on them," it said.

Johnson told police the boy ate dinner, watched a movie and later was "sleeping and breathing fine." But at 8 a.m. June 25, his brother told Johnson that the victim not breathing. Medics pronounced him dead at the scene.

The Ramsey County medical examiner said the boy died of a closed head injury with internal bleeding in the head and spinal cord and bilateral bleeding in the optic nerve sheaths. He also had suffered a depressed skull fracture.

According to the complaint, the child's mother told officers that he had fallen off playground equipment twice within the past couple of weeks. Both incidents occurred in Johnson's care.

After the first fall, the doctor cleared the boy, she said. She didn't take him to the doctor after the second fall, the complaint said.

In an earlier, unrelated incident, the child hit his head on the bed rails about a month ago, the complaint said. A doctor examined the child on May 22 and diagnosed two head contusions. At that time, "medical records indicated that the child had a good appetite and ate a turkey sandwich 'with vigor,' " according to the complaint.

An autopsy found that the boy had recently suffered a severe skull fracture with fresh bleeding on the left side of his head and bleeding to the right side of his clavicle.

The medical examiner determined that he would not have survived those injuries more than 24 to 48 hours. The death was ruled a homicide.

The investigation revealed a similar incident involving Johnson and another child. In 2013, he was the focus of an unrelated child protection case in Dakota County.

According to that report, Johnson got into an altercation with a woman who was his girlfriend at the time as she held her 1-year-old child. The woman said Johnson choked her and punched the baby in the face, causing a black eye.

Child protection service investigators said maltreatment had occurred, but the woman did not press charges.

When investigators confronted Johnson with that finding and about the "timing of [the 3-year-old's] injuries," he showed little emotion, the complaint said.

"He provided no explanation for the severe injuries that took the child's life," it said.

Investigators spoke with the victim's mother about the "severity and timing of [her son's] injuries and that they could not be explained by the accidental falls."

The mother "cried and confirmed that [her son] was just fine when she left him in the defendant's care on the evening of June 24," the complaint said.

Karen Zamora • 612-673-4647

Twitter: @KarenAnelZamora