The pickup truck that collided with a tour van near Yellowstone National Park last week is to blame for the crash, which left seven people dead, according to police.

Idaho State Police said a Dodge Ram pickup truck crossed the center line on Highway 20 near Henry’s Lake State Park and hit the Mercedes tour van, causing both vehicles to burst into flames. It is unclear what caused the truck to leave its lane.

“Crash reconstruction is ongoing, and ISP troopers are working to determine all contributing factors,” state police said on Tuesday.

The driver of the truck and six of the 14 people inside the tour van died in the crash, police said.

Roger Merrill, a 60-year-old man who was driving home when he saw the vehicles on fire, told The Associated Press that bystanders were caring for the van survivors on the side of the highway until first responders arrived.

First responders arrive at the scene after a deadly collision between a pickup truck and tour van near Henry’s Lake State Park in eastern Idaho on May 1. (Roger Merrill via AP)

“It took an unnervingly long time for help to arrive just because of the location,” Merrill said.

He also said he frequently sees tourist vans on Highway 20 as it leads to Yellowstone National Park.

The fatal crash happened in eastern Idaho, about 16 miles west of Yellowstone National Park in Wyoming. (Getty Images)

“We extend our deepest condolences to the families and loved ones of those lost in this tragedy,” ISP Sgt. Blake Higley said. “We are incredibly grateful to the passersby who stopped to help and to the first responders whose quick actions and teamwork aided those in critical need. This was a challenging and emotional scene for everyone involved.”

The highway was closed for almost seven hours while first responders and the Idaho Transportation Department managed the scene and cleared the roadway.