Rescue crews in Radford say it could take days to dig out two women missing in the rubble.
Search dogs tracked them down, but it's not known if they're still alive.
The body of a third worker was found near the core of the blast.
The cause of the explosion is not known, but a natural gas leak is being investigated.
A union representative says the plant was evacuated last week because of a gas leak. The official says it was "a ticking time bomb waiting to happen."
The company maintains the plant was safe.
The damage has been estimated to be in the $30-50 million range.
New River Castings and a related parts manufacturer, Lynchburg Foundry, are owned by Troy, Mich.-based Intermet Corp., which employs about 1,000 people in Radford, a town about 30 miles west of Roanoke.
The two plants devastated by the blast make transmissions and other parts for the auto industry.