A portion of a four-lane bridge collapsed Saturday night, killing at least three people and sending cars and a tractor-trailer rig plunging into the Hatchie River, authorities said.

Police did not know how many people were injured.A section of the 55-year-old bridge apparently was on top of the rig, said Denise Jones of the Tennessee Emergency Management Agency. The driver was still missing hours after the collapse.

Emergency workers in rescue boats were having a hard time reaching the rig and one of the cars, and divers were called to the scene.

"They haven't got to the rest of the vehicles yet. They've just got to the one on top," said Capt. James Hubbard of the Covington Fire Department.

Authorities are investigating what might have caused the collapse along U.S. 51 near Covington, which is about 45 miles north of Memphis.