PORTLAND — Maine wardens say an all-terrain vehicle carrying a man and three children flipped over, killing a 19-month-old boy.

Wardens say the ATV toppled backward Sunday while ascending a hill on private property in Windham.

Wardens say the man’s toddler son was taken to Maine Medical Center, where he died Monday.

The identities of the young victim and his father weren’t immediately released. The Maine medical examiner’s office planned an autopsy Monday.

The Maine Warden Service will be working with the district attorney as the investigation unfolds. Windham police and Cumberland County deputies assisted.

Wardens say they don’t believe helmets were being worn.