GUNTER, Texas — Transportation officials worked Thursday to determine why two freight trains collided head-on, killing an engineer and injuring four other crew members in a fiery wreck that left cars scattered across the tracks.

A major rail line north of Dallas remained closed. U.S. Department of Transportation officials were on the scene early Thursday along with National Transportation Safety Board investigators, and railroad officials were also en route to the rural crash site about 50 miles north of Dallas.

More than 20 rail cars lay in accordion fashion on the tracks after the trains collided about 6 p.m. Wednesday.

At least one of the locomotives burned and flames spread to grass and other nearby vegetation. The engineer who died, who was on the southbound train, was found about 50 feet from the wreckage. Names of the dead and injured were not immediately released.

One of the injured men suffered severe burns and was taken by medical helicopter to a Dallas hospital, said Texas Department of Public Safety Trooper Rebecca Uresti. The other three injured crew members were in stable condition at Wilson N. Jones Medical Center in Sherman, said hospital spokeswoman Karis Schirmer.

Joe Faust, a spokesman for Fort Worth-based Burlington Northern Santa Fe Corp., said both trains belonged to the company.

Rail cars on one of the trains were empty, while the other train was hauling rocks, the Grayson County sheriff's department said. No one was evacuated, but a hazardous materials crew was called to clean up spilled diesel fuel.

The collision is the second this month in Texas. On May 3, two freight trains collided on a Union Pacific track just south of downtown San Antonio, injuring three people, derailing some cars and spilling about 5,600 gallons of diesel fuel into the San Antonio River.