As many as 100 cars and trucks slammed into each other in thick fog on an interstate Tuesday in fiery, chain-reaction collisions that killed at least seven people and injured 50, authorities said.

The fog rolled out of the Hiwassee River and onto Interstate 75 at about 10 a.m. EST, causing a tractor-trailer rig to jackknife "and then everybody else just plowed right into the fog bank," said Cecil Whaley of the Tennessee Emergency Management Agency.