GEORGETOWN, Colo. -- When fire broke out deep underground at a hydroelectric plant in the Rockies, officials at the surface dropped a radio down to five trapped men in a tunnel and were relieved to learn they were OK.

But by the time emergency crews reached them six hours later, they were dead.

On Wednesday, a day after the tragedy more than 1,500 feet underground at Xcel Corp.'s Cabin Creek power plant, investigators struggled to figure out what went wrong.

It was unclear whether the five maintenance workers were burned, suffocated or overcome by fumes from the highly flammable epoxy sealant they were using to coat the inside of the empty, 12-foot-wide water pipeline.

Officials waited for air quality tests to be completed Wednesday before going in to retrieve the bodies and gather evidence. The first body was removed at about 3:45 p.m.

Authorities defended their rescue efforts, saying smoke, the complexities of the 4,000-foot tunnel's design and uncertainties about the dangers prevented them from going in after the men for more than 3 1/2 hours after the blaze broke out.

"We didn't know what was causing the fire, what was feeding the fire," Undersheriff Stu Nay said. "You never know, when you're dealing with airflow and the intensity of the fire where we're facing a backdraft situation, what we're running into."

He added: "It's dangerous work. We can't afford to have someone else go in and complicate the problem."

The blaze erupted when a machine used by the workers to coat the tunnel caught fire, Xcel Energy spokeswoman Ethnie Groves said Tuesday. But exactly what burned -- the machine, the epoxy or both -- was unclear, she said.

Nine employees of RPI Coating of Santa Fe Springs, Calif., had been sealing the inside of the pipe to prevent corrosion, a routine procedure that followed an annual inspection. The tunnel delivers water from an upper reservoir to turbines at the lower reservoir that generate electricity at the plant.

The smoldering fire broke out about 1,400 feet from the tunnel's bottom and was reported at about 2 p.m., authorities said.

Four RPI workers escaped from the tunnel and were treated at a hospital and released. Five others scrambled about 1,000 feet above the fire but were trapped by smoke and the steep, nearly impossible-to-climb slope at a spot where the tunnel bends from a 10-degree angle to a 55-degree one, Nay said.

Officials dropped a radio to the workers, who reported at about 2:12 p.m. that they were uninjured. Sometime between 3 p.m. and 3:30 p.m. was the last contact rescuers had with them, Nay said.

Rescuers also dropped breathing masks and air tanks into the tunnel but were unsure if the workers were able to find them or use them, Nay said. Powerful fans were used to drive air into the tunnel and clear it of smoke so that the trapped crew members could breathe.

Alpine rescue team rapellers prepared to enter the tunnel from the top, but were called off for fear of toxic fumes and because it would be difficult to get the victims out through the steeply pitched top end of the tunnel, Nay said.

Officials said they decided it would be easier to reach the trapped men from the bottom of the tunnel, which can be reached via an elevator.

A crew from Colorado's Henderson molybdenum mine, specially trained in confined-space rescues and firefighting, began making its way through the smoke in the tunnel at 5:40 p.m., officials said. At 8:10 p.m., the crew reached the trapped men, discovered they were dead, and retreated, Nay said.

The federal Occupational Safety and Health Administration and the U.S. Chemical Safety Board are investigating.

The agencies will focus, among other things, on conditions inside the confined space and what type of protection and safety training the maintenance crew had, OSHA spokesman Rich Kulczewski said.

"We're devastated over the loss," said RPI Coating spokesman Marc Dyer. "They were very experienced guys. They were some of our best."

The names of the dead were withheld until their families could be notified.