Petaluma police on Monday afternoon evacuated several homes and closed a block of Ely Boulevard South after a landlord found a World War I bomb shell.
Previous tenants apparently left behind the antique device in the garage and the landlord called police at about 2 p.m. after finding it, Police Lt. Tim Lyons said.
Officers closed Ely between Caulfield Lane and Weaverly Drive for about five hours as they waited for bomb experts to remove the shell.
The Sonoma County Sheriff's Office bomb squad determined the device was a type of artillery shell made in 1913 and used during World War I.
But the exact type of ordinance wasn't clear and its threat unknown. Deputies called bomb disposal technicians from Travis Air Force Base in Fairfield to remove the device.