The fire at a North Carolina fertilizer plant continued to burn Wednesday morning, a more than 30-hour stretch, forcing thousands to evacuate their homes as authorities warned that potentially combustible chemical compounds stored on-site could cause an explosion.

The blaze began Monday evening at the facility in Winston-Salem, and it continued uncontrolled into Tuesday. Parts of the site were still actively burning Wednesday morning, officials said. The flames forced firefighters to retreat, and officials urged the evacuation of about 6,500 people who live within a mile of the plant, which is operated by the Winston Weaver Co. No injuries have been reported.

The “possibility of explosion has not gone down from yesterday,” Winston-Salem Fire Department Division Chief Bobby Wade said at a news conference Wednesday — despite first responders’ previous assessment that the explosion risk would be lower by this point.

The chemical compound in question is ammonium nitrate, which Winston-Salem Fire Chief Trey Mayo on Tuesday called “somewhat unpredictable.” It’s used to make fertilizer and can fuel fires in certain conditions, including heat exposure. There was an estimated 500 tons of it at the site, Mayo said, and an additional 100 tons in a rail car next to the facility. That’s far more than the amount of ammonium nitrate that caused an explosion at a fertilizer plant in West, Tex., in 2013, he said. That blast killed 15 people, injured 252, and damaged or destroyed 500 buildings.

“We have at least three times as much product on hand here as they had in West, Texas,” Mayo said. “I don’t know how much more compelling I can be to try to make people understand the seriousness of this situation.”

Wade said in the early-morning update on Wednesday that it was not clear when residents would be able to return to their homes. He warned that cold temperatures were causing the smoke from the fire to lie low and advised people with respiratory problems to stay indoors.

Firefighters expected it to take time for the fire to fully burn itself out, Wade said.

Wake Forest University, which is near the plant but mostly outside the evacuation zone, canceled classes and relocated students who live in an apartment complex within the one-mile radius.

Andrew Carroll, a spokesman for Winston Weaver, said that all 36 employees are safe and that the company is cooperating with authorities.

“We will continue working with first responders and relevant officials to ensure the safety of the community, and we will participate fully into the investigation into the cause of the fire,” Carroll said in a statement.

INCIDENT ALERT - Structure fire 2400 Block of Cherry Street. Fire attack in progress. #WSFire .107 pic.twitter.com/54NOgxJZxs

Hundreds of emergency workers, from local and federal agencies, have responded to the scene. The first firefighters arrived around 7 p.m. Monday after being called in to deal with a fire on the plant’s loading dock. Once there, they found a large fire that had spread to the building itself, parts of which were up to 80 years old, officials said.

The structure became engulfed in flames and collapsed. Firefighters left the site after trying to contain the blaze for about two hours, Winston-Salem Battalion Chief Patrick Grubbs said during an earlier update. The risk of explosion was too great, and they couldn’t get enough water to the site, he said.

Officials do not know what caused the fire, and an investigation is ongoing.

A small town in Texas. A huge explosion. An unsolved mystery.

Ammonium nitrate is a chemical compound of white salt and an oxidizer that fuels fires by supplying oxygen.

It has a “lot of variabilities in the way it can be mixed,” Rachel Lance, a biomedical engineer and assistant consulting professor at Duke University, said in an interview. When used in fertilizer, it is mixed with components meant to discourage explosion, Lance said. A blast can still happen, but there is a lower risk.

“We typically only see explosions in fertilizer blends when there’s a large fire that initiates it,” Lance said, pointing to the Texas explosion as an example. That can occur when it reaches a “temperature that’s sufficient to cause an explosion.”

Lance, who specializes in patterns of trauma from blasts, underlined that evacuation in a situation like this is critical because if it triggers an explosion, many can get injured from flying glass.

What we know about ammonium nitrate, the chemical compound linked to the Beirut explosions

Matthew Smith, a member of a North Carolina hazardous materials regional response team who spoke at the Tuesday morning briefing, said the “one-mile radius is the worst-case scenario. I feel perfectly confident in the one-mile radius.” Officials said they would probably preserve the perimeter until Wednesday evening.

The Environmental Protection Agency was on the scene Wednesday morning, and authorities have been monitoring the air outside of the evacuation radius. Officials warned residents that smoke and poor air quality was likely but implored them to call 911 only if there is an emergency.

Drone footage shared by the city early Tuesday showed heavy yellow-tinted smoke still billowing from the charred facility.

The evacuation area around 4440 North Cherry Street has almost 6500 residents or 2,497 households. #MapForsyth#CityofWSFire pic.twitter.com/h1DcwA1YDk

The city said the education building at the nearby Winston-Salem Fairgrounds is available as a shelter for affected residents. Officials told those who evacuated to prepare to be away from their homes for at least 48 hours because firefighters do not plan on returning to the site before Wednesday evening.

Once they’re able to safely access the facility, authorities and company staffers will begin to assess the extent of the damage. If no explosion occurs, the city will have dodged a major disaster, said Mayo, the fire chief.

“When I learned how much ammonium nitrate was on site last night,” he said, “I felt as uneasy at a fire scene as I’ve ever felt in 33 years in this business.”