A fiery wreck on a bridge that spans the U.S.-Mexico border killed at least four people, police said.

The accident Thursday on the American side of the 3.2-mile Pharr-Reynosa International Bridge involved two tractor-trailers and three other vehicles, Pharr polie Lt. Lupe Salinas said.

The two tractor-trailers were headed in opposite directions when the wreck occurred shortly after 7 p.m., he said.

Three people died after their pickup truck plunged from the bridge and landed on a road below, Salinas said. Another person died in a collision with one of the tractor-trailers.

Rescue workers at the bridge were searching for more possible victims and investigating the accident, Salinas said. The victims have not been identified.

Pharr Emergency Management Coordinator Elsa Sanchez said there appeared to be other injuries, but none was life-threatening.

The pickup that fell from the bridge had Texas license plates, Sanchez said. Both tractor-trailers and a minivan involved in the wreck appeared to have Mexican license plates, she said.

U.S. Customs and Border Protection spokesman Felix Garza said the bridge was closed in both directions and crossers were being diverted.

Fred Brouwen, assistant bridge director, said the wreck caused no damage to the structure of the bridge.

The bridge is typically closed from midnight to 6 a.m. Brouwen said he did not expect the bridge to reopen until 6 a.m. on Friday.

The bridge, which has one southbound lane and three northbound lanes, is Texas' fourth-largest commercial vehicle crossing with Mexico, Brouwen said. About 1,700 trucks and 7,000 cars cross the bridge from Mexico to Texas each day. The bridge handles about 6,000 cars and 1,600 trucks heading south each day, he said.

"This is basically the second accident in the 15 years we've had the bridge," Brouwen said.

Pharr is in the Rio Grande Valley, about 225 miles south of San Antonio.