MEXICO CITY (AP) — A fiery crash between a tanker truck carrying fuel and a passenger bus Saturday killed at least 18 people in northern Mexico, authorities reported.

Prosecutors in the northern border state of Tamaulipas said the crash might have been caused by one of the truck's two tank trailers coming loose.

The collision left both vehicles completely burned. Police photos showed the bus was reduced to a tangle of smoking, charred metal.

Tamaulipas state police initially found nine sets of remains, but by early afternoon prosecutors said nine more had been recovered. The death toll could rise, they said.

The crash occurred before dawn on a highway that leads to the northern city of Monterrey.

The driver of the fuel truck apparently survived and is under investigation. The bus had apparently set out from the central state of Hidalgo and was headed to Monterrey.

The semi-truck was towing two tank trailers in tandem. Such double-container freight trucks have been involved in numerous deadly crashes in the past.

Because weight restrictions and safety inspections in Mexico are lax, there have been calls in the past to ban the extremely heavy and unwieldy trucks.

State prosecutors said in a statement that "the crash may have occurred because one of the fuel tanks came loose."