State safety regulators in southern Nevada on Tuesday began investigating a fatal construction accident in a water supply tunnel being built at Lake Mead, the latest in a series of mishaps and setbacks at the multi-million dollar project that began in 2009.

A Nevada Occupational Safety and Health Administration investigator visited the 3-foot-diameter tunnel where one man was killed and another injured about 4:30 p.m. Monday approximately 600 feet below ground near the Lake Mead National Recreation Area.

"State OSHA is investigating," Carrie Foley, executive assistant to the state agency's director, said Tuesday. "I can't answer any other questions right now because it is an open investigation."

Thomas Albert Turner, 44, of Henderson, and another worker were alone in a segment of the tunnel when they were hit by exploding grout material after some material became loose, officials for the Southern Nevada Water Authority said.