ESTES Park, Colo. -- An air tanker crashed in rough terrain on Thursday while battling Colorado's latest wildfire, killing both crew members on board, fire fighting officials said.

A fire information officer declined to identify the dead crew members. He said all air tankers in the Rocky Mountain region were immediately grounded for 48 hours, a standard procedure.

Ryan Powers, an official from the company that operated the plane, Greybull, Wyoming-based Hawkins and Powers, was quoted as saying the PB4Y-style aircraft had been built in 1945.

"The mission these planes were designed for was to be a very low-flying aircraft," he said.

A month ago, an air tanker operated by the same company -- which specializes in fighting wildfires -- crashed while fighting a fire in California, killing three on board.

The National Transportation Safety Board was expected to immediately begin an investigation of Thursday's crash, Powers said.

Witnesses reported seeing a plume of smoke at the site where the plane was fighting the Big Elk Fire near Estes Park, about 50 miles (80km) northwest of Denver.

The crash sparked a one-acre fire. Fire fighters immediately tried to put it out before it could spread.

The Big Elk Fire grew to 1,200 acres (490 hectares) on Thursday from 500 acres (200 hectares) the day before when it erupted on steep terrain.

The fire was human caused, but investigators have not yet determined if it was arson. the fire was about one-to-two miles (1.5 km to 3 km) from homes.

The fire was the latest to hit Colorado in what has been a heavy fire season due to drought and dry conditions in forests throughout the west.