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KEY LARGO (CBSMiami) — OSHA cited a South Florida utility company and a contractor after investigating the deaths of three workers in a manhole.

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Robert Wilson, 24, Elway Gray, 34, and Louis O’Keefe, 49, died back in January 2017 after succumbing to toxic gases while working in a Key Largo manhole.

Related: Men Working Underground In Key Largo Collapse, Die

Authorities say this was a chain reaction. One worker went into the manhole and collapsed. A second worker who went to rescue him also collapsed. The same thing happened to a third worker who also tried to help. The same thing happened to a firefighter who tried to save them.

OSHA’s investigation found safety failures led to their deaths. Testing during their investigation found that there were lethal levels of hydrogen sulfide and carbon monoxide underground.

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The findings prompted OSHA to cite Douglas N. Higgins, Inc. and its related contracting company, McKenna Contracting with 10 “serious” violations totaling $119,507, in penalties.

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The violations include failing to purge and ventilate the confined space, exposing workers to an asphyxiation hazard and not providing necessary rescue and emergency equipment to employees.

“The hazards of working in manholes are well established, but there are ways to make it safe,” said Condell Eastmond, the OSHA area director in Fort Lauderdale. “Three employees needlessly lost their lives and others were injured due to their employer’s failure to follow safe work practices.”

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Click here to view all for the citations.