An electrical accident caused a Friday night barn fire near Casper that destroyed a helicopter and sent three firefighters to the hospital, a fire official said Saturday.

One Natrona County firefighter and two Mills volunteer firefighters were sent to the Wyoming Medical Center on Friday night after possible exposure to cyanide, county Fire Inspector Mike Lindahl said.

"Cyanide pellets are used by most ranchers to control prairie dog populations," Lindahl said. "The firefighters were exposed during the mop-up stages because some of the packages were still smoldering."

Natrona County Fire Captain Matt Keating said the three firefighters were treated and later released. They suffered no permanent damage.

The fire began at 5:30 p.m. on Friday and crews from the Casper and Natrona County fire departments, as well as Mills volunteers, had the blaze fully contained by 11 p.m.

"Crews went on autodefensive when they arrived here because of the nature of the fire," Lindahl said. "We had a water relay system and were successful saving two of the four barns."

The fire started in the middle of a series of connected barns that housed a helicopter, two cars, a four-wheeler and ranch equipment, Lindahl said, pointing at the remains of the blackened barns on 6 Mile Road, where the fire began.

"Crews entered the first structure as it began to burn until explosions began in the second barn and they had to come back out," Lindahl said.

Everything was destroyed, and Lindahl said he did not know the damage estimate.

Contact city reporter Christine Robinson at (307) 266-0639 or christine.robinson@trib.com