BUTTE, Mont. -

A fire engine and a pickup truck collided Thursday night causing an explosion. It killed the Three Forks fire chief who was driving the fire truck and a Helena family of five, including three children, in the pickup truck.

The Montana Department of Transportation closed U.S. Highway 12 east of Helena for hours to give troopers time to investigate.

Helena resident John Elliott was headed home on Highway 12 Thursday night when he witnessed the crash. He said he pulled to the side of the road to get a better look.

"By the time I got out of my vehicle, it was pretty much just going up in a ball of flames. I didn't even know what I was looking at at first," said Elliott.

Elliott soon realized it was fire truck and pickup truck.

"A few minutes after I got out of my car that's when I heard the first explosion," said Elliott.

Elliott said he and others watched in shock.

"Nobody saw anybody walk out of this one," said Elliott. "And that's when I knew it was pretty serious."

Sergeant Alex Betz with the Montana Highway Patrol was one of many on the scene late Thursday.

"They were off the shoulder of the road, fully engulfed in flames," said Betz.

Investigators measured skid marks to recreate the scene and figure out what happened before the crash.

"There were troopers on scene from the origination of the call until this morning," said Betz.

Betz said they determined the fire truck was headed east and the pickup was headed west. Both vehicles ended up in a ditch on the north side of the road.

The Department of Transportation reopened the road around 8 a.m. after all the evidence of the crash had been collected.

"There's a lot of information that we pick up off the roads," said Betz.

Elliot said it took time to process what saw Thursday night. "That helpless feeling that you can't do anything to help them, it hit me later," said Elliott.