Two people were killed today when their firefighting air tanker crashed while fighting a 5,000-acre blaze along the Nevada-Utah border, an NTSB official tells ABC News.

The plane - a heavy air tanker - went down shortly before 1 p.m. in the Hamblin Valley roughly two hours west of Cedar City, Utah, said Chris Hanefeld, a fire information officer with the Bureau of Land Management. Hanefeld said the plane had been dropping fire retardant.

Accident investigators with the NTSB are on the way to the crash site and should arrive tonight, NTSB spokesman Nicholas Worrell told ABC News.

The victims were not identified and the cause of the crash is unknown.

Hanefeld said the tanker that crashed is a P-2V, owned by Neptune Aviation Services of Missoula, Mont.

The age of the aircraft that went down is not known.  The P-2 was originally developed for the Navy in the 1940s.

The tanker was working the White Rock fire that started Friday in Lincoln County, Nev., about 25 miles north of Caliente. High winds pushed the flames across the border into Utah. Officials believe the fire was sparked by a lightning strike. No structures are threatened.

While the cause of this accident is unknown, the age and shrinking size of the nation's fleet of firefighting air tankers has been the subject of renewed concern among lawmakers in fire-prone states. Sen. Ron Wyden, D-Ore., this week criticized the U.S. Forest Service for not moving faster to modernize the agency's contracted fleet of large air tankers.

Forest Service chief Tom Tidwell told ABC News this week the agency is working to modernize the fleet.

"I would tell the public that with the combination of the large air tankers along with the helicopters and firefighters is that we are ready and we are prepared to deal with this coming fire season," Tidwell said.