Associated Press

WINSTON-SALEM — The explosion threat from a fire at a  North Carolina fertilizer plant has “greatly diminished” now that much of a combustible chemical has burned off and firefighters have returned to the site to spray down what’s left, the city’s fire chief said Thursday.

Still, Winston-Salem Fire Chief Trey Mayo said local officials were still urging people who evacuated to stay away because some explosion threat remains.

Mayo said that with the help of experts on the fertilizer ingredient ammonium nitrate, officials determined it was safe for firefighters to get close enough to set up more unmanned sprayers early Thursday at the site. Firefighters had worked the blaze after it broke out Monday night but had to pull back due to the threat of the explosion. Since then, it had largely been allowed to burn, with officials monitoring from the air and fire teams on standby. Rain early Thursday also helped with cooling effects.

“The threat of an explosion was greatly diminished overnight through the cooling operations,” Mayo said at a news conference Thursday afternoon.

Firefighters also inspected a nearby railcar containing ammonium nitrate and determined it had not caught fire. Firefighters and heavy machine operators are now ready to start on a systematic in-person effort to make sure all of the fire is out across the site. That effort could take days.

Mayo said that they were still recommending that evacuated people stay away in case the ongoing firefighting effort jostles ammonium nitrate and introduces air that could cause it to engulf again.

The area where officials have called on people to evacuate includes about 6,500 people in 2,500 homes, officials have said. Mayo declined to estimate when people could return.

The fire began Monday night at the Winston Weaver Company fertilizer plant on the north side of the 250,000-person city and quickly consumed the entire building. No injuries were reported. An estimated 500 tons of combustible ammonium nitrate were housed at the plant and nearly another 100 tons of the fertilizer ingredient were in the adjacent rail car.

Specialists from several states are involved and air monitoring equipment around the scene is being checked for different gases, fire officials said.

Officials initially thought the situation could end in 36 hours, maybe even two days. But Mayo says it could still take days longer to finish putting it out.