SUPERIOR, Wis. (AP) — Toxic fumes apparently overcame four workers in an industrial well Thursday, killing them before rescuers could reach them, authorities said.

It was not known what the workers were doing in the well, which was estimated to be about 20 feet deep, when rescuers were called, Fire Chief Tad Matheson said.

The well contained water and toxic hydrogen sulfide fumes, commonly released with sewer products, Matheson said. 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."

No other details were available, but the victims were believed to be employees of the company that owns the site, authorities said. A sign near the site identified the business as Lakehead Blacktop Demolition Landfill.

Battalion Fire Chief Richard Rugg said all four bodies had been recovered.