Colorado ? Federal investigators focused Thursday on working conditions and safety procedures inside the hydroelectric plant tunnel where five workers died after a fire erupted on the equipment they were using.

Autopsies were pending, but authorities believe smoke and fumes from Tuesday’s fire killed the workers as they sat trapped deep underground. Communication from the crew ended about an hour after the fire broke out.

The bodies, found scattered along a 200-foot length of the 12-foot-wide pipe, didn’t have burn marks, indicating that the men probably died from the smoke and fumes from the chemical fire, Undersheriff Stu Nay said.

OSHA was looking at what kind of protection and safety training the maintenance crew had; safety procedures for the type of work being performed inside the tunnel; and documentation that safety procedures were being followed, agency spokesman Rich Kulczewksi said.

The OSHA probe was expected to take months and involve federal, state and local agencies, said the agency’s Denver area director, Herb Gibson.