In a statement, Clear Creek County sherrif’s office said the maintenance workers had difficulty getting the anti-corrosion mixture of paint and epoxy sealant through spray tubes. The workers added solvent into a mixing hopper to thin the mixture but the resulting change in temperature of the chemicals caused a heater in the mixer to activate, leading the fumes to ignite.

The fire started approximately 1400ft (430m) up the penstock from the lower portal of the penstock, near Lower Cabin Creek Reservoir. Five workers were working above the seat of the fire, and despite their efforts to climb away up the gradual slope, moving 1000ft (just over 300m) away, penstock fans being reversed to drive smoke away and breathing apparatus being lowered from the top – they were found dead by emergency crews.

All five bodies were taken to the coroner’s office in Jefferson County for postmortem examination. The official cause of death has not been released but examiners confirmed that the workers had not suffered burns or external injuries.

Four workers who were below the location of the fire escaped through the lower portal, and they had medical checks. In total, the maintenance contractor RPI Coating of Santa Fe Springs, California, has 11 workers on site, two of the group being outside the penstock. The contract was to perform preventative maintenance on the penstock.

The accident is under a joint investigation by the sheriff and coroner of Clear Creek County, the Colorado Bureau of Investigation, the US Chemical Safety Board, and the US Occupational Safety and Health Administration.

Cabin Creek pumped storage plant was built at approximately 10,000 ft (just over 3000m) in the Rocky Mountains and started operations in 1967.

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