(Updates death toll, adds details)

MEXICO CITY, April 14 (Reuters) - Twenty-four people were killed when a bus and tractor-trailer crashed and burst into flames on Saturday in the Mexican state of Chihuahua close to the U.S. border, authorities said.

Eduardo Esparza, spokesman for the state attorney general's office, said 18 others were treated in hospitals for burns after the fire reduced the two vehicles to ashes.

"All of them had burns, some were totally cremated," Esparza said.

Authorities said the victims were take to a morgue in Ciudad Juarez, across the border from El Paso, Texas. Family members gave blood samples to help identify bodies.

A police official said most of the dead were passengers on the bus, which slammed into the truck before both vehicles were engulfed by flames.

The official, who asked not to be identified, said the crash may have been caused by excess speed, or possibly because the bus driver fell asleep.

The bus driver, who was carrying at least 40 people including two conductors, was not accompanied by a relief driver.

In Mexico, long-distance buses normally travel with at least two drivers.

The attorney general's office has begun an investigation into the cause of the accident and the subsequent fire. The flames may have been fed by diesel fuel from the vehicles, the office said in a statement.

It did not say if the vehicles' drivers were among the dead.

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