Toxic fumes are believed to have killed four workers in Superior, Wis, whose bodies were pulled from a 20-foot deep well near a blacktop demolition company, authorities said.

Battalion Fire Chief Richard Rugg said rescuers recovered the bodies Thursday night. The victims are believed to be employees of the company that owns the site, although it was not immediately known what they were doing in the well, authorities said.

A sign near the site identified the business as Lakehead Blacktop Demolition Landfill. There was no activity visible there Friday morning. A person who answered the phone at a listing for Lakehead Blacktop declined to comment.

Douglas County authorities received a call at about 5:30 p.m. that there were people in the well. Fire Chief Tad Matheson said the well contained water and hydrogen sulfide fumes, which commonly are released with sewer products.

The victims apparently were overcome, he said, although autopsies would have to be done to determine the cause of their deaths.

"They had a heavy enough toxic atmosphere that they would be overcome by the fumes, but there was water down in the well," Matheson said.

Hydrogen sulfide gas is poisonous, flammable and colorless and gives off a strong odor of rotten eggs, according to the Centers for Disease Control and Prevention. At high concentrations, people can lose the ability to smell it, making it extremely dangerous. Brief exposure to high concentrations can cause difficulty breathing and loss of consciousness.

Firefighters with breathing equipment led the recovery effort, and workers pumped water out to help.

The well is outdoors, but the gas is heavier than air, Matheson said.

"So the gas would just stay in that well area," he said. "If you're below ground, that's where the fumes would accumulate."

The victims were believed to have died before rescuers arrived, Matheson said. Their names were not immediately released.

Authorities said they believe the well is within the village of Superior in northwestern Wisconsin, but planned to use Global Positioning System to make certain.

The village is about 150 miles north of Minneapolis and has a population of 500 people. It is adjacent to the city of Superior, a port city that has a population of about 27,000.