Two workers were killed after explosions and a large fire tore through an Ohio industrial recycling building early on Aug. 18, prompting a massive emergency response, officials said.

Firefighters were dispatched at around 4:50 a.m. local time after receiving 911 calls reporting an explosion and heavy fire at Brent Industries in Toledo, Ohio, according to Toledo Fire and Rescue Chief Allison Armstrong. When crews arrived, Armstrong said they encountered heavy flames and smoke.

“Part of the building had already collapsed,” the fire chief said at a news conference. “Very challenging conditions for both the employees to evacuate and the fire and rescue department.”

Crews found the bodies of two employees, a man and a woman, inside the building hours later, according to Armstrong. Two or three other employees sustained minor injuries and were treated at the scene.

Toledo Mayor Wade Kapszukiewicz said a shift change was underway at the time of the explosions. The incident prompted an unusually large response because of the size of the structure, hazardous materials on site and reports that workers were trapped inside the building, according to officials.

By the afternoon, portions of the building were still burning and smoldering as firefighters worked to contain the fire and prevent it from spreading to nearby businesses and structures. Armstrong said dangerous conditions at the site, including the risk of more explosions and structural collapse, complicated firefighting efforts.

The Toledo Fire and Rescue Department is working to determine the cause of the explosions.

Air monitoring finds no immediate public threat

The U.S. Environmental Protection Agency said it was notified of the incident through the National Response Center at around 9 a.m. local time and deployed personnel from Ann Arbor, Michigan, to assist with air monitoring and environmental assessments.

“When we arrived shortly after, the fire of the tanks was pretty much extinguished,” EPA on-scene coordinator Jon Gulch said at the news conference. “So at that point we were left with just the white smoke, and we’ve done air monitoring around and in the plume.”

Air monitoring detected no hazardous levels of contaminants, indicating no immediate threat to the surrounding community, according to Gulch.

Company faced citations following fatal Alabama explosion in 2015

Brent Industries, which is based in Brent, Alabama, provides textile reconditioning and industrial recycling services to help companies reduce waste and save money, according to its website. The company was founded in 1977 and operates in Brent and Toledo.

In 2015, one worker was killed and several others were injured after explosions and a fire occurred at the company’s Alabama manufacturing plant, reported The Tuscaloosa News, part of the USA TODAY Network.

Investigators with the Occupational Safety and Health Administration found the explosion occurred in an industrial laundering and dry cleaning operation that used mineral spirits, a petroleum-based solvent, to clean shop towels and gloves, records show.

The agency subsequently cited Brent Industries for nine serious workplace safety violations involving flammable liquids, hazard communication, exit routes and protective equipment. It issued a fine that was later reduced to $27,300, according to records.

USA TODAY reached out to Brent Industries and OSHA for comment.