Officials are investigating the death of a 4-month-old boy who was found unresponsive in a Chelsea apartment Monday.

The infant was discovered by his mother in the Washington Avenue apartment around noon and was taken to Whidden Memorial Hospital, where he was pronounced dead, according to the Suffolk district attorney’s office.

According to Chelsea authorities, a preliminary examination of the boy’s body did not reveal any immediate signs of violent trauma, but Chelsea police and State Police are investigating. The Office of the Chief Medical Examiner will conduct an autopsy to determine the cause of the child’s death.

“Any time a child dies, we want to perform a complete and thorough investigation,’’ said Jake Wark, a spokesman for the district attorney’s office.

A neighbor who lives across the street from the large brick home said Monday evening that she saw the mother crying and holding the child.

“There was no blood,’’ said the neighbor, who declined to give her name.

According to Wark, there were no other children in the household. He could not say whether the Department of Child and Family Services had been contacted.

Officials are asking anyone with information to contact Chelsea police detectives or the State Police detectives unit in Suffolk County.

Aimee Ortiz can be reached at aimee.ortiz@globe.com. Follow her on twitter @aimee_ortiz.