SCOTTSDALE, Ariz. -- Two workers died after apparently getting overcome by toxic fumes in a sewer outside a Scottsdale shopping mall Monday, authorities said.

Scottsdale Fire Department officials say three workers were doing some maintenance or repair work in the sewer near The Promenade around 4 p.m. but weren't wearing full protective gear, for an unknown reason.

A bystander heard screams coming from the sewer and called authorities, Scottsdale Fire Division Chief Jay Ducote said.

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One of the workers was able to climb out of the 15-foot sewer despite feeling lethargic, but the other two workers apparently were rendered unconscious by fumes inside, according to authorities.

Reporter Lindsey Reiser of CBS Phoenix affiliate KPHO-TV said, "The stench coming from that sewage line at times was extremely overwhelming, even in the open air."

Ducote said arriving crews recorded high levels of hydrogen cyanide and hydrogen sulfide coming out of the sewer.

"Hydrogen sulfide is a really bad chemical that is lethal at 250 parts per million and it was about 200 parts per million with the hazmat teams got out there," Ducote said.

It wasn't immediately clear if the men were city workers or employed by a private company, authorities said, and their names weren't immediately released.

The lone survivor was taken to a Scottsdale hospital for evaluation.

Firefighters in full hazmat suits ventilated the 3-1/2 foot manhole cover for hours before crews started to go down into the sewer, using a tripod with harnesses to recover the bodies.

Crews also set up a decontamination tent for firefighters after the job was completed.