As a fire at a fertilizer plant in Winston-Salem, N.C., burned for a third day on Wednesday, officials said they would indefinitely maintain a one-mile voluntary evacuation zone around the plant and were ordering firefighters to keep their distance because of the risk of an explosion.
About 600 tons of ammonium nitrate were at the Winston Weaver Company plant when the fire broke out on Monday night, prompting the authorities to close a school and circulate in the streets to encourage residents in more than 2,000 households to go to a shelter.
On Wednesday, Bobby Wade, the division chief of the Winston-Salem Fire Department, said firefighters were evaluating the scene but were not on site at the plant, where the possibility of an explosion “has not gone down,” Chief Wade said.
“It is unsafe for anyone to be on that site,” he said at a news conference.
Those conditions prompted officials to extend the voluntary evacuation period, which had been tentatively scheduled to end Wednesday morning, until “the risk of an explosion has passed,” the city said in a statement.
The fertilizer plant lies in a neighborhood with about 6,500 residents, city officials have said. The city’s fire chief, Trey Mayo, said at a news conference on Wednesday that it was not clear how many people had heeded the recommendation to evacuate, and that it was not logistically possible to force people from their homes and businesses.
That would require patrols in a wide zone where emergency personnel could be in danger themselves, he said. “Short of going door to door, which is really not practical, we have saturated the area with messaging,” he said.
Gov. Roy Cooper of North Carolina also urged people to leave. “The No. 1 concern is to make sure that we save lives and to make sure that people don’t get hurt,” he said at the news conference.
Although the evacuation was not mandatory, officials have spent the past two days trying to emphasize how dangerous the event could become. Chief Mayo compared it to a fire at a fertilizer plant in Texas in 2013, in which 240 tons of the same product exploded, destroying or severely damaging nearly 200 homes and killing 15 people.
He said on Wednesday that the amount of ammonium nitrate at the Winston-Salem site was enough to generate “one of the worst explosions in U.S. history.”
It was not clear what caused the fire, but officials from the Winston Weaver Company were providing investigators with information. The Bureau of Alcohol, Tobacco, Firearms and Explosives said on Tuesday that federal investigators were assisting local fire departments.
Denise D. Adams, a Winston-Salem City Council member who represents the North Ward district that includes the plant, said in an interview on Tuesday She said she was watching television on Monday night when she heard sirens. At around 7 p.m., she recalled, “there was a loud shaking explosion, boom, and immediately my mind thought when I heard it — fertilizer plant.”
The blaze started at a loading dock, she said, according to preliminary information.
Firefighters who initially responded to the blaze on Monday were later ordered back because the building where the ammonium nitrate was stored had collapsed, limiting their ability to get enough water into the area, Chief Mayo said.
North Hills Elementary School, which is within the evacuation zone, was closed to in-person learning for a second day on Wednesday. Wake Forest University also canceled classes again on Wednesday. The university said it would provide alternative housing to students and faculty and staff members who live inside the evacuation zone.
Chief Mayo said personnel teams, helicopters and drones were evaluating the fire. Rain was in the forecast for Wednesday night through Thursday, which he said was a “generally positive turn of events.”
No injuries or deaths have been reported. All 36 employees who worked at the plant have been accounted for, Chief Mayo said. One who was at the plant on Monday night was the first to call 911, he said.
Ms. Adams, the City Council member, said that the plant had undergone an inspection in December, and that “there was no noncompliance” issue at the plant.
She said a shelter had been opened at a nearby fairgrounds.
The Occupational Safety and Health Administration, the federal agency that regulates workplace safety and the storage of chemicals, told the fertilizer industry in a letter after the disaster in Texas that the explosion of bulk ammonium nitrate took place 20 minutes after the first report of a fire.