BRYAN, Texas -- Officials evacuated more than 50,000 residents in Brazos County and Bryan for hours because of a fire at a chemical plant believed to contain explosive ammonium nitrate.

The fire broke out about noon Thursday at the El Dorado Chemical Co. plant on Highway 21.

Bryan-College Station fire dispatcher Andy Throne said no injuries were reported from the fire, where fertilizer and other chemicals are blended and packaged.

Brazos County 911 district associate director Patrick Corley said a large evacuation was ordered because of concern from a toxic cloud.

"This covers the entire city of Bryan and the entire northern half of Brazos County, so it's a considerable area," Corley told KPRC Local 2 at 4 p.m.

Two people had to be rescued from their homes because they had no way out.

"I was left at the place by myself. I wasn't scared because I didn't really know what was happening," said Coletha Hall, 89, who lives near the plant.

By 7 p.m., the evacuation area was reduced to a two-mile radius around the plant.

The following roads remain closed.

Highway 21 West from Harvey Mitchell to Highway 47
Leonard Road from Harvey Mitchell to Highway 47
Sandy Point from Harvey Mitchell to Old San Antonio Road
Old San Antonio Road from Sandy Point to Highway 21 West
Luza Lane at Highway 21 West

All other areas of Brazos County and the city of Bryan were reopened to residents. The fire was extinguished by 7:30 p.m.

At least 12 people were treated at area hospitals for respiratory problems, but none was life-threatening.

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.

Between 1,000 and 5,000 people were expected to remain away from their homes Thursday night because they lived near the plant.

Texas A&M said it was prepared to accommodate the evacuees overnight.

"It's not mandatory. We know some people just can't get out, so in those cases we're imposing the shelter-in-place concept, where we're asking people to turn off their air conditioner, shut their windows, don't go outside, bring the pets in," Bryan city manager David Watkins said.

Residents said roadways were jammed with people trying to get out of the evacuation area.

"Traffic is so bad trying to get out of town, " resident Bob Nixon said. "There's a small smell in the air, but not anything that's real noticeable. But there is like a very light smog-haze where we're at, which is about 4 miles from the fire."

Nixon said a lot of businesses appeared to have closed early to allow employees time to evacuate.

Texas A&M canceled classes for the remainder of Thursday.

"As far as battling the fire, they're letting it burn at this point and trying to evacuate people as far as possible from it," Corley said.

"If you mix water with this chemical, you can have some undesirable results," Watkins said.

"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.

Officials said ammonium nitrate can especially cause problems for those with respiratory issues.

"It will irritate your lungs. You don't want to be breathing it and that's why we're suggesting people leave the area," Watkins said. "Don't panic, but take it seriously."

El Dorado Chemical Company issued the following statement through its spokesman, Brent Gooden.

"While we are thankful no injuries have been reported, we deeply regret the enormous inconvenience this incident has caused residents and businesses in Bryan and College Station as well as other nearby communities. The well-being of the public, first responders and our employees is top of mind with us at this time. In that regard, we appreciate the thorough and responsible way in which local authorities responded. In addition to dispatching our safety team to assist and conducting our own comprehensive internal review, we will also work with local, state and federal officials to support them in every way we can as they conduct their investigations."

The cause is under investigation.

"We don't know what caused it for sure," Watkins said.

Hazardous materials units will go to the warehouse on Friday to clean up the remains.

The plant is located about three-fourths of a mile west of Bryan.