LAS VEGAS >> A construction worker was killed and another worker was injured in a tunnel under construction at Lake Mead after some material became loose and pressure sent grout flying through the air.

The two workers were alone in a segment of the 3-foot-diameter tunnel near Lake Mead National Recreation Area when they were hit by the exploding grout material late Monday afternoon.

The second worker’s condition was unknown, said Southern Nevada Water Authority spokeswoman Nicole Lise. The workers were not identified.

The tunnel is part of a troubled effort to drill a third drinking water supply line to the Lake Mead reservoir. The multi-million-dollar project has been beset by flooding and cave-ins since construction began in 2009, and work has been delayed by about two years.

Workers in other areas of the tunnel were not affected by Monday’s accident. Early media reports that more than a dozen workers were trapped in a collapsed tunnel or exposed to toxic fumes were false, the water authority said.

Las Vegas depends on Lake Mead for about 90 percent of its drinking water. Construction on the third tunnel began amid concerns over the Colorado River reservoir’s shrinking supply. The third intake is designed to keep water flowing to Las Vegas even if drought shrinks the lake below the level of the two existing conduits.

The new tunnel, bored through solid rock beneath Lake Mead, will be 3 miles long.

“During the assembly of some segments of the tunnel ring, one of the segments became loose, and the grout material exploded and hit two workers,” Lise said.

Engineers were inspecting the site late Monday, and it was unclear when construction might resume.

The project was delayed last year after a cavern 600 feet underground filled with water and muck. In another delay, some mining equipment was lost to flooding.