BRYAN, Texas -- An evacuation ordered after a chemical plant believed to contain explosive ammonium nitrate caught fire has been completely lifted, KPRC Local 2 reported Friday.

The fire broke out about noon Thursday at the El Dorado Chemical Co. plant on Highway 21, about three-fourths of a mile west of Bryan.

The company blends and packages fertilizer and other chemicals.

"The best information we have is that the fire was started by some welding," said John Salsman with Brazos County emergency management.

Brazos County 911 district associate director Patrick Corley said a large evacuation was ordered because of concern from a toxic cloud. The evacuation area included the entire city of Bryan and the entire northern half of Brazos County. About 50,000 people were affected.

Residents in much of the evacuation area were allowed to return to their homes at about 7 p.m. Thursday. The evacuation area was reduced to about a five-square-mile area around the plant, affecting about 1,000 people.

Results of air quality testing showed that there is no longer a threat to residents. Everyone was allowed to return home.

Highway 21 and feeder roads near the plant will remain closed indefinitely, but residents who need to use the closed roads to access their homes will be allowed to do so.

About 34 people were treated at hospitals for smoke inhalation and/or eye irritation.

The company has set up a toll-free number for anyone who has needs that resulted from the fire. Residents may call 800-959-2658 to submit claims.

Hot spots flared up throughout the night, but firefighters couldn't put water on them.

"It's a very unstable product. It's water reactive. If we were to go in and try to put water on it, it could intensify the fire and even cause an explosion," Bryan Fire Department Chief Mike Donoho said.

Bryan Fire Department Assistant Chief Randy McGregor said officials are trying to decide how much of the dangerous chemicals are left.

"It was reported to us that we started with about 570 tons of product, so it was a pretty large amount," he said. "We know there's still a pretty good pile there still smoldering, but we're not sure exactly how much is left."

Officials said the warehouse was destroyed.

Reed Arena at Texas A&M was opened as a shelter for those who needed a place to go. For those with pets, Pearce Pavilion cared for the animals until the emergency was over. Officials had enough food for about 300 people, but about 1,000 people were at the shelter. Local restaurants donated food to make sure everyone got something to eat.

Officials with the Texas Commission on Environmental Equality were doing an environmental assessment of the site Friday.