The victims' conditions weren't immediately known, and officers were searching the building for the shooter, said Troy police Lt. Gerry Scherlinck.
Police in bulletproof vests and with guns could be seen going into the building while people from inside walked quickly out through a parking lot.
The office building is about 15 miles north of Detroit.
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Gladewater police responded after a trailer carrying lumber crashed into a business Wednesday morning.
This morning, Tropical Depression Harvey developed in the southern Gulf of Mexico and continues to drift northward as it gradually strengthens.
The Mount Pleasant Fire Department is holding a fundraiser for a law enforcement officer.