Story highlights Sheriff: Three construction workers died after being exposed to toxic gases

A firefighter who attempted to save the men was also hospitalized

(CNN) The job should have been routine. Tasked with checking out an underground pipeline beneath a newly paved road, a Florida utility worker lifted a manhole cover and climbed down.

He didn't make it back alive.

Neither did another worker who went to check on his colleague -- neither did a third man who went in after them both.

The three construction company employees died from a toxic combination of gases in the underground pipeline in Key Largo, according to the Monroe County Sheriff's Office . A firefighter who attempted to save the men also collapsed seconds after entering the manhole.

He was brought back to the surface and flown to a hospital. Three sheriff's deputies also were taken to the hospital.

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