After an investigation, officials now say it was a combination of fog and smoke from large piles of burning dead trees that caused a closure of Interstate 84 near Boardman. It also triggered accidents that injured 7 people.

The East Oregonian reports the Boardman Tree Farm has numerous piles of what's called 'hog fuel' that spontaneously ignited late last week, sending clouds of dense smoke around the area. The tree farm is located a little over three miles from the interstate near Boardman.

Fire crews, said the Oregonian, likened it to a haystack fire, with lots of white smoke creating poor visibility. This smoke, combined with area fog, actually shut down the freeway near Boardman. Monday, at least four semi-trucks and three passenger vehicles crashed in the dense fog, sending 7 people to the Good Samaritan Hospital in Hermiston. Officials say an air ambulance was dispatched, but couldn't get into off the ground due to the fog. Several of the people were taken to Portland area hospitals.

Fire crews have located the exact burn areas, and have been able to keep them from spreading, but the piles are so large, they cannot be extinguished. Officials say they will have to burn themselves out. There's still smoke and fog in the area, but the materials are expected to extinguish themselves in a few days.