The Los Angeles Police Department's bomb squad investigated a device that apparently fell from a truck and onto the side of the westbound Foothill (210) Freeway, police said.
The device was found just east of Roxford Street at 4:07 p.m., said Officer Mike Lopez.
"We will be treating it as if is something, but hope that it is nothing," Lopez said.
The device appeared to be a replica of the famed "Fat Man" nuclear bomb.
The freeway was closed between Polk and Yarnell streets while police investigated the device, Lopez said.