UPDATED 7:28 p.m.

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.

The Winston-Salem Fire Department tweeted that the voluntary evacuation for a 1-mile (1.6-kilometer) radius around the plant would be reduced to 660 feet (200 meters) Thursday evening.

Earlier on Thursday, Winston-Salem Fire Chief Trey 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 at the site.

Firefighters had worked the blaze after it broke out Monday night but had to pull back because of 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.

The fire began Monday night at the Winston Weaver Co. 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 (450 metric tons) of combustible ammonium nitrate were housed at the plant and nearly another 100 tons (90 metric tons) of the fertilizer ingredient were in the adjacent rail car.

There are specialists from several states involved and air monitoring equipment around the scene is checking 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.

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