The Major Case Squad was called to Madison Friday to investigate a body found in a car trunk.
Officers have not released details about the victim, including the person's gender and cause of death.
Fairview Heights Police Lt. Michael Hoguet said the case is connected to a person reported missing in St. Louis.
Hoguet said officers are conducting a suspicious death investigation.
The car, a 1996 Ford Taurus with Missouri license plate UJ2-D5X, was reported to be abandoned at Third and Jackson streets in Madison.
Madison Police found the body in the trunk shortly after 9 a.m. Friday.