Hundreds of residents were allowed to return to their homes hours after a fire broke out at a nearby hazardous waste incineration plant, but investigators have not determined its cause.

About 1,500 people were initially evacuated as thick smoke rose from the plant in southern Arkansas early Sunday. No one was injured, but residents reported hearing a series of explosions and one person said it rattled the windows of his home.

Doug Riley, vice president of operations for El Dorado-based Teris LLC (search), said the fire was out by Sunday afternoon, "except for one small area in the storage building."

The plant disposes of hazardous waste chemicals from industries and government operations, including mixed solvents, different types of gases and a variety of batteries.

The company, along with fire officials, agreed to let the blaze burn out rather than send firefighters into the hazardous area, Riley said.

After U.S. Environmental Protection Agency (search) officials tested air and soil in the area, many people were allowed to return to their homes Sunday night. More than 500 residents had to spend the night elsewhere, El Dorado Police Capt. David Smith said.

Chris Ruhl, the EPA's on-scene coordinator, said the air quality was fine for people outside the evacuation zones. He wouldn't comment on the air inside the zones, saying the EPA was continuing to monitor air quality along with state officials.

Authorities advised people not to use their air conditioners and to keep their doors shut as a precaution while the air quality was being monitored.

Shelters were set up at churches and community centers for people displaced from their homes, including residents of nursing homes who did not need to be transferred to hospitals.

Herman Hill and his wife planned to stay at a church shelter. He said his asthma had been bothering him since the thick cloud of smoke began covering the city.

"We need some face masks," said Hill, 49. "We didn't seal off our house or our air conditioner. Will the chemicals get into our food? I'm worried."

About 150 inmates from the county jail were evacuated to a school gymnasium before being moved to other jails Sunday night, Smith said.