A well accident killed four workers who apparently were overcome by toxic fumes Thursday and died before rescuers could get to them, authorities said.

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

Douglas County authorities reported receiving a call about 5:30 p.m. that there were people in a well at the industrial site in the village of Superior.

The victims were believed to be employees of the company that owns the site.

The well contained water and toxic hydrogen sulfide fumes, which commonly are released with sewer products, Fire Chief Tad Matheson told The Associated Press.

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

He said he didn't know what the workers were doing in the well, which he estimated at around 20 feet deep.

A sign near the site identified the business as Lakehead Blacktop Demolition Landfill.

Hydrogen sulfide gas is poisonous, flammable and colorless and gives off a strong odor of rotten eggs, according to the Centers for Disease Control. 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.

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

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

Family members and friends gathered across the street from the landfill during the recovery effort.

The victims were believed to have died before rescuers arrived, the chief said.