BOSTON (AP) – A police bomb unit, alerted by a pedestrian to something suspicious on the side of the street, found a hand grenade used in military training early Wednesday.

The device was working and had the potential to explode, said Elaine Driscoll, a police spokeswoman.

“It was absolutely a potential threat,” she said.

The device was first spotted on the side of a street in a commercial area around 7 a.m. Wednesday. By the time police arrived, someone had placed a plastic cover from a trash barrel over it.

Police and agents from the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives then conducted a sweep of the neighborhood, looking for additional devices, but had not found anything after several hours.

Driscoll said police do not know who left the device there.