A couple from Georgia and their 12-year-old son were killed Thursday when a tractor-trailer rear-ended their car on the New Jersey Turnpike in Mount Laurel, state police said. Their 18-year-old daughter survived.

The crash closed the southbound lanes for hours.

Police said the family was at mile marker 39.9 in a silver Toyota Camry about 10:10 a.m. when traffic in their lane suddenly slowed and the tractor-trailer slammed into the car, crumpling it.

Killed were the Toyota's 51-year-old driver, his 49-year-old wife, and their son, police said. The daughter had minor injuries.

Lt. Stephen Jones, a state police spokesman, said the family was from Stone Mountain.

Jones said investigators were trying to determine why traffic slowed just before the crash five miles north of Exit 4. The truck also hit a gold Ford Edge, but no one in it was injured.

A second tractor-trailer ran off the road to avoid hitting the pileup, Jones said.

The investigation is continuing, and no charges have been filed.