York Region police have safely deactivated a hand grenade found behind a retail plaza in Richmond Hill on Wednesday morning.

"A bomb disposal expert deactivated (the explosive) ... at about 12:15 p.m," Const. Laure Perks told CTV.ca.

The bomb squad unit was called to the plaza at 460 Elgin Mills Rd., west of Bayview Avenue, just after 10 a.m. when a worker discovered the item in a box at the rear of the building.

Police closed off nearby streets and cleared businesses in the area while they investigated the incident.

Officers quickly determined the grenade was real and the explosive was deactivated by hand, without incident. A robot was not needed, Perks said.

Police reopened the streets soon after.

Investigators are not sure where the device came from or who may have placed it there.

The grenade is possibly a Second World War vintage, police said.

Officers haven't determined if the device contained explosive material.

Perks said the detonating device at the top had been modified to simulate a cigarette lighter.

With a report from CTV's Jim Junkin