HELENA, Mont. -

An investigation into a June crash involving a fire truck and pickup with a family of five a mechanical failure on the fire truck.

"Our investigators and reconstructionists have invested countless hours into this crash," said Montana Highway Patrol Deputy Chief Greg Watson.

According to the Montana Highway Patrol, the drive train -- a system of parts that delivers power to the driving wheels -- of the fire truck snapped, causing the right wheel to lock up and forcing the truck into oncoming traffic.  Investigators say the driver of the pickup attempted to veer right but couldn't avoid the oncoming fire engine.

Investigators found that marks on the road helped point investigators in the right direction.

"There's several markings on the pavement from where the drive shaft was spinning and colliding with the pavement," said Watson.

The impact caused the a fuel tank on the fire truck to catch fire. 

"The impact ruptured a diesel fuel tank on the passenger side of the fire truck, which resulted in a wide dispersal of diesel fuel that caught fire soon after impact," said Watson.

The report said Fire Chief Todd Rummel died from smoke inhalation while unconscious. Matthew Boegli, Crystal Ross and their three young children died from blunt force trauma.  Police say none of the children were restrained. 

"Whenever someone dies on the highways we patrol, we take that very seriously. Even personally," said Watson.

Investigators said the fire truck, a 2002 International 750-gallon truck underwent a recall for a drive train issue in 2005. MHP says the drive train should have been inspected and undergone maintenance based on that recall, but could not say whether the maintenance was completed.

Rummel was driving the vehicle about 55 mph back from Helena where one of its pumps were had undergone maintenance.