A Michigan man using a magnet while fishing in a river made an unusual catch -- a German grenade from around World War I.

Joseph Alexander said he was fishing in the Grand River near the Sixth Street Bridge in Grand Rapids when his magnet pulled up a device later identified by police as a German Granatenwerfer mortar.

"I thought it looked like a grenade, but not one I've seen before," Alexander told WOOD-TV. "We brought it back home, took some pictures of it, posted it online, and we started getting comments that 'it's a grenade, call the police.' So that's what we did."

The Grand Rapids Police Department said the device is being safely stored and will be detonated using C4. Police said the mortar is likely not very dangerous due to being submerged for a long period of time, but authorities are being cautious with the explosive.

Alexander said he has been magnet fishing for about a month and has pulled up some knives, but the grenade is his "first cool find" while practicing the hobby.

"I didn't think anyone would throw a live grenade in the river," he said. "And for a second there, I was questioning if I was even right."