Police have closed down parts of Maryland Route 295 and Interstate 195 in Anne Arundel County as they search for a man armed with a shotgun or rifle.
Nobody has been injured, and police have not yet determined if shots have been fired.
Investigators believe the man may have used a hammer to break out the windows of a parked car in the area of the two roads at about 11:30 a.m.
The suspect is described as a white man between 50 and 60 years old wearing a black and red flannel shirt and blue jeans.
Maryland State Police and Anne Arundel County police are searching for the man with helicopters, police dogs and officers on foot, authorities said.
This item has been updated.