Tracy police and the San Joaquin County Sheriff's Department bomb squad were called out, and they found what they thought was a training grenade with an explosive fuse.
Crews blew up the grenade inside the garage at 1433 Duncan Drive.
They also found a large amount of gunpowder, which they burned on the street outside the home, because they said it was unsafe to transport.
The home was in the news a few months ago when a city inspector called police about a "houseful of ammunition." The bomb squad determined that the guns were locked in fireproof safes.
This was the same foreclosed home where code enforcement officers found 23 sick cats and three dead ones — and ordered the home's occupant to leave.