The Boeing 741-R was flying from Chicago's O'Hare International Airport to John F. Kennedy International Airport in New York when it declared an emergency and landed safely at Detroit Metropolitan Airport.
The flight crew had reported engine trouble after one of the freighter's four engines apparently fell off its wing mount.
Michigan State Police had received no reports of debris on the ground, WDIV-TV, Detroit, said.
A Federal Aviation Administration spokeswoman said there were unconfirmed reports the engine fell into Lake Michigan.
A team from the National Transportation Safety Board was to inspect the plane on the ground in Detroit.