Emergency officials in Nevada are responding to a tunnel collapse near Lake Mead National Recreation Area that may have trapped a crew of construction workers.

A spokesman for the Southern Nevada Water Authority says it's unclear how many workers were affected by the Monday afternoon accident.

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

The 23-foot diameter tunnel will be 3 miles long through solid rock beneath Lake Mead.

Las Vegas depends on the Colorado River reservoir for about 90 percent of its drinking water.