CHICAGO (AP) — Lithium batteries exploded loudly overnight inside a burning former paper mill in northern Illinois that officials had believed was long abandoned, and fire officials have decided to let the blaze burn out because they fear trying to extinguish it could trigger more explosions.

The fire that started in Morris Tuesday prompted city officials to order the evacuation of 3,000-4,000 people in some 950 nearby homes, a school, church and small businesses.

On Wednesday, as thick, black smoke continued to billow from the building, Police Chief Alicia Steffes said the evacuation order would remain in place until at least 9 p.m. and “might be extended.”

Police are stationed throughout the area to prevent people from entering, although anyone who can prove they live there may return to retrieve essential medicine, she said.

Fire Chief Tracey Steffes said thus far air quality tests were “coming back favorable,” but he cautioned that changing weather conditions and other factors could cause the air quality to deteriorate.

Mayor Chris Brown urged anyone experiencing respiratory problems to contact their physicians.

The fire chief said he’s gathering information from fire departments and other experts on how to fight the fire in a building that — to the surprise of his department and other city agencies — was being used to store nearly 100 tons of lithium batteries ranging in size from cellphone batteries to large car batteries.

Steffes’ firefighters stopped using water on the blaze minutes after they arrived when they discovered the batteries because water and firefighting foam can cause batteries to explode. And he said while he has heard some ideas on how to battle the blaze — road salt has been suggested — he won’t send crews to battle the fire because of the unknowns about what’s inside.

“I don’t know 100% what was stored in that building, only what they’re telling us what was stored in that building,” he said.

Further, Steffes said that while his department and other agencies have fought fires at buildings that contain lithium batteries, he had thus far found nobody with fires that involve so many batteries. He said the battery explosions overnight could be heard across the city.

The mayor said the city didn’t know the building was being used to store batteries until it caught fire, and that he knows very little about the company that owns them.

“The name of the company is Superior Battery…and we didn’t know they existed until yesterday afternoon,” said Brown. Apparently nobody else at City Hall did either, because there’s no record of a business license or any communication between the company and any city department, he said.

Barely concealing his anger at the very serious danger his firefighters were in, Steffe suggested that he couldn’t trust any information coming from the company as a result.

“We had no way of knowing they were doing business…there,” said Steffes, adding a company official told him they had occupied the building for about a year. Steffes said the paper mill had been vacant for decades.

No information was immediately available on Superior Battery. The fire chief said company representatives were not invited to Wednesday’s news conference.

The mayor said the police department will conduct an investigation about the storage of the batteries and that other agencies, including the state fire marshal and the Illinois Attorney General’s office, have already been contacted.

Morris is about 70 miles southwest of Chicago.

The Morris fire came two weeks after an explosion and massive blaze at a chemical plant near Rockton, an Illinois town along the Wisconsin border, forced the evacuation of hundreds of homes for several days. Nobody at the plant or the surrounding community was injured by the June 13 fire that officials later determined was started accidentally during maintenance work.

By DON BABWIN for the Associate Press