A propane-fueled auxiliary heating unit pressed into service because of the extreme cold exploded Sunday morning outside an apartment complex in Rogers. No one was injured.

Residents in The Preserve at Commerce, a 192-unit complex at 13600 Commerce Blvd., were knocked off their feet, glass from shattered windows was embedded in opposite walls and people from as far away as a mile said their houses shook, Rogers Police Chief Jeff Beahen said.

The initial explosion happened about 8:45 a.m., and subsequent explosions and fireballs reportedly four- to five-stories high continued for another 35 to 40 minutes, Beahen said, until the fuel valve was located and turned off.

All 192 units — 400 to 500 residents — were evacuated until building inspectors could say that the building was safe. Some residents drove off. Others were housed at the Hampton Inn, across the street, then moved to the nearby Rogers 18 movie theater. About 125 residents were there enjoying movies until midafternoon, Beahen said. The Red Cross and Salvation Army were assisting residents.

A maintenance man working the pool room told police and fire officials that he was thrown backward by the force of the explosion. Other residents said they were knocked off their feet or out of bed. A dozen or more calls came in from residents as far as a mile away saying their houses shook.

There was “fairly significant structural damage” to the walls in part of the complex. Windows were blown out on the north and east side. Interior walls may have shifted, Beahen said.

The complex’s 5,000-pound propane tank, buried in the ground, did not explode, Beahen said. The problem occurred in the unit above the propane tank. It converts LP gas into gas that can be used to help heat the huge complex, which has an indoor pool and meeting rooms.

The apartment complex has an “interruptible contract” with CenterPoint Energy to help reduce the load on natural gas lines. The utility has such contracts with hundreds of large buildings and business — such as some industrial plants — that have an alternative heating source. In return for using the alternative heat source when demand is at its highest, the customers get a break on their rates. The contracts are entirely optional, said CenterPoint spokesman Becca Virden.

The contract customers are usually only asked to interrupt their service for a couple of days at a time when temperatures drop precipitously low.

“It’s not based on specific temperatures but on demand,” Virden said. “Each customer is different.”

Beahen said 10 or 12 apartments won’t be habitable until repairs are made. He said he and Fire Chief Brad Feist are 90 percent certain that other residents will be allowed back into their apartments by Sunday evening.

The cause of the explosion is not known yet. The state fire marshal’s office and the Office of Pipeline Safety were expected to be at the scene Sunday to investigate.