UPDATED 6:35 p.m.
An uncontrolled fire at a fertilizer plant in North Carolina forced thousands of people to evacuate as firefighters stood back Tuesday because of the danger of a large explosion.
Authorities drove through neighborhoods and knocked on doors urging residents to leave within a one-mile (1.6-kilometer) radius of the Winston Weaver Company fertilizer plant on the north side of Winston-Salem, where the fire began Monday night. Overnight, bright orange flames and thick plumes of smoke could be seen shooting into the sky. No injuries were reported.
"We heard the explosion. It shook our house," said Michelle Shepherd, who evacuated from her home near the plant. "We weren't sure what it was. I opened up my front door and the entire sky was nothing but orange. I could see the flames shooting over the trees."
The city's fire chief said the fire had been "relatively static" overnight, but with 600 tons of combustible ammonium nitrate stored at the site, the risk of an explosion would remain through Wednesday.
"I've been in this business 33 years and when I learned how much ammonium nitrate was on site last night, I felt as uneasy at a fire scene as I've felt in my 33 years in this business," Winston-Salem Fire Chief Trey Mayo told reporters.
Wake Forest University, most of which lies just outside the evacuation zone, canceled classes and urged students in dormitories to stay indoors with windows closed.
The area included about 6,500 people in 2,500 homes, the Winston-Salem Fire Department said.
At least 90 firefighters fought the fire for about 90 minutes after it was discovered at a loading dock around 7 p.m. Monday, but the risk of an explosion forced them to retreat, Mayo said. The fire quickly consumed the entire building and it collapsed. An unmanned fire truck was left behind to continue pumping water onto part of the site.
Drones and a helicopter monitored the fire from above, and teams of firefighters were on standby, letting the fire burn for now, the chief said.
Mayo told reporters an estimated 500 tons of ammonium nitrate were housed at the plant and another 100 tons of the fertilizer ingredient were in an adjacent rail car. He said that is more of the chemical than was present at a deadly blast at a 2013 Texas fertilizer plant blast that killed 15 people.
"So if that doesn't convey the gravity of the situation and how serious folks need to take it, I don't know how else to verbalize that," he said.
Mayo said the chemical generally needs to be in a confined space to explode, so the risk will depend on whether the material is stacked deep enough for the top layers to put enough pressure on the bottom layers.
Authorities warned of smoke and poor air quality in the city of about 250,000. Matthew Smith, a hazardous material expert with a regional state task force, said the gases released by the blaze are more of an irritant than something that could cause serious harm, barring an underlying lung condition.
A representative of the Winston Weaver plant issued a statement Tuesday that no workers were injured or killed in the explosion. It thanked firefighters and first responders.
Richard Van Erp, who lives blocks from the fertilizer plant, said late Tuesday morning that so much smoke was billowing into his apartment complex's parking lot that he couldn't see the cars at times.
Van Erp, who doesn't have a car, said he had called a non-emergency police number to ask for a ride to a shelter and was waiting to be picked up.
"It takes a lot to spook me, and I will say I'm pretty shaken right now," he said. "They said that, more than likely if it blows, the evacuated area will be leveled completely. I'm not even a quarter of a mile away."
Norvell McDowell, who lives less than half a mile away, was awakened early Tuesday to sirens and first-responders telling him to seek shelter elsewhere. The 68-year-old double amputee who uses a wheelchair said he was aware of the fire before going to bed, but didn't know the urgency of the situation. He was taken to a shelter opened at the Winston-Salem Fairgrounds.
"We woke up about 3:30 and they came through the parking lot with sirens," McDowell said.
Shepherd, the evacuee who felt her house shake, said she used a phone app to listen to emergency scanner traffic and heard firefighters talking about abandoning their equipment.
"That's when I started thinking we needed to evacuate," she said, adding that several hours passed as she and her partner debated whether to relocate.
"All of sudden, there were three more, almost consistent, explosions and we could feel the house rock again," Shepherd said. "I said, 'You know what? I'm not even going to take a chance. We need to go."