Prosecutors in Michoacan state say the car holding the remains of the three young men was found on a roadside on Christmas Eve.
The other two victims were Mexican men aged 21 and 24. All three were from or had family in the nearby village of Quiringuicharo, Michoacan.
An employee of the prosecutors' office who was not authorized to be quoted by name said Tuesday that the U.S. teen has been identified as 18-year-old Alexis Uriel Marron.
The office had no information on his home town, but the Chicago Tribune reported he was from the Chicago area.