An explosion east of Casper launched a pup trailer full of butane half a mile down Interstate 25 on Tuesday.

The small trailer exploded into a fireball estimated at several hundred yards around noon and burned for several more hours.

It was called a boiling-liquid-expanding-vapor explosion, according to Natrona County Sheriff's Lt. Stewart Anderson, as he waited for the gas to finish burning in the main trailer. Even from a half a mile away, it was the largest explosion of this kind he has ever seen.

Nobody was injured in what Anderson called a "mushroom cloud." Hours later, all emergency personnel still waited half a mile away for the fire to calm, Anderson said.

He's still not sure what caused the truck to crash. It was pulling a tanker and pup trailer and carrying more than 12,500 gallons of butane. No other vehicles were involved in the wreck.

Passing motorists helped the PC Transport driver, 77-year-old Paul Prime, out of his tractor after the crash, according to the Wyoming Highway Patrol. He was wearing his seatbelt and wasn't injured.

Law enforcement officials closed I-25 between Hat Six Road east of Casper and Glenrock. Traffic was rerouted onto Highway 20/26 until about 4:30 p.m. when officers opened northbound lanes, according to the Wyoming Highway Patrol.

When crews responded to the fire, officers realized the liquified gas in the tanker could release all at once and cause a massive explosion.

"We told everyone to back off. On these types of incidents, you have tremendous amounts of fire and heat as well as flying debris," Anderson said, referring to the flying pup trailer.

In 26 years working for Natrona County, Anderson said he'd seen smaller explosions similar to this one, but only in training videos has he witnessed something this large.

"It could have been much worse," he said. "Nobody has been hurt and nobody was killed."

People in Casper may not have seen the fire because it was in a low part of the interstate, but Anderson said smoke could be seen from town.

The explosion also burned roughly 60 acres of grassland.

Crews responded from Natrona County, Evansville, Casper and Glenrock fire departments, as well as Converse County and Natrona County emergency management and the Wyoming Highway Patrol.

According to the Wyoming Highway Patrol, the load of butane was traveling from Casper to its final destination in Douglas. The truck and trailer were destroyed.

By 5 p.m. Tuesday, Anderson said crews were working to remove the rest of the butane from the main trailer, clear the truck and debris from the southeast lanes and reopen the southbound lanes of the interstate.

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