About a week after a 4-month-old boy was found unresponsive by his mother in a West Palm Beach, Florida, home, the boy's 12-year-old uncle handed a note to a police officer.

"I killed my nephew."

New details emerge from a newly released West Palm Beach police report about the preteen charged with negligent manslaughter in the infant's death. The minor told police he put a pillow over the baby's face to stop his cries.

The Palm Beach Post is naming neither the preteen nor the infant because of their ages.

On Dec. 2, the 4-month-old's mother left the boy and other young children alone in the care of her 18-year-old sister and her 12-year-old brother before noon, according to police.

When the mother came home just before 7 p.m., she went to see her son and found him sleeping on his stomach. Then she noticed his blue lips and that he wasn't breathing, police said. The baby's mother attempted CPR and called 911. The 4-month-old was taken to St. Mary's Medical Center, where he was pronounced dead.

Investigators note the infant was being treated for a respiratory infection at the time.

On Dec. 8, the preteen went up to a Riviera Beach police officer while at school and said he wanted to "tell him something serious but would have to write it down."

After the student handed his confession over, the officer asked him what he meant. The 12-year-old explained his nephew wouldn't stop crying so he put a pillow over the boy's face.

On May 23, the Palm Beach County Medical Examiner released its final report stating the cause of the death was asphyxia. The manner of his death was ruled homicide, according to the report. This month, a warrant was issued for the preteen's arrest and he was placed into the juvenile facility July 27.

Kroll also said he may not have any unsupervised contact with minors under the age of 10.