HELENA (AP) — Investigators have concluded a collision between a fire engine and a pickup truck that killed six people in Montana was caused by a mechanical failure in the fire engine, a state Highway Patrol official said Tuesday.

The break in the 2002 International fire engine’s front drivetrain, which powers the wheels, caused Three Forks Fire Chief Todd Rummel to lose control while driving at 55 mph on U.S. 12 about 10 miles southeast of Helena on June 19, Lt. Col. Greg Watson said.

“The mechanical failure caused one of the wheels to lock up, which traveling at that speed would make it very difficult for the driver to steer the vehicle,” Watson told reporters at a news conference.

The fire engine veered into the path of a 1997 Chevrolet pickup truck carrying a family of five traveling in the opposite direction. The driver of the truck tried to avoid the fire engine, but the vehicles collided on the shoulder of the highway.

The crash punctured the engine’s fuel tank, spraying diesel across the area that ignited, and fire engulfed both vehicles, Watson said.

Air tanks on the fire truck heated and exploded, he said.

A team of six investigators spent nearly two months trying to piece together what happened. Their work was made more difficult by the lack of survivors and the devastation of the wreckage.

They determined that Rummel died of smoke inhalation while unconscious. Matthew Boegli, Crystal Ross and their three young children died of blunt-force trauma on impact, Watson said.

The adults were wearing seat belts, but a 4-year-old boy and 3-year-old twins sitting in the back seat of the truck’s cab were not, he said.

Several parts of the front drivetrain failed in the fire engine, and investigators were not able to determine which broke first, he said.

Rummel was driving the fire engine back to Three Forks from Helena, where it had been undergoing repairs to its water system. The mechanical problems that led to the crash were not related to the recently completed repairs, Watson said.

Three recalls were issued for International 4800 models manufactured in 2002 like the fire engine Rummel was driving, according to a National Highway Traffic Safety Administration database.

One recall issued in 2003 was for an excessive driveshaft angle that could accelerate its failure, causing it to “become separated from the vehicle without warning and fall into the path of other vehicles on the highway,” the recall notice read.

The Three Forks fire department’s truck had either passed inspection or undergone repairs for the problem listed in the recall, Watson said.

Watson referred questions about whether either driver was impaired to Jefferson County Sheriff County and Coroner Craig Doolittle, who did not immediately return a message.

The highway patrol does not anticipate filing any charges in the crash, Watson said.