Los Angeles  - Federal officials were set to begin an investigation Monday into the deadly crash of two medical helicopters which collided in mid-air Sunday on the approach to a hospital in Flagstaff, Arizona.

Six people were killed and three injured in the Sunday collision as the two medical helicopters plummeted to the ground and set fire to a wooded area,

Teams from the National Transportation Safety Board and the Federal Aviation Administration were expected at the site Monday and open a probe into the crash.

"The NTSB and FAA are involved as well as our own investigators," Sergeant Tom Boughner of the Flagstaff Police Department told KPHO-TV in Phoenix.

A spokesman for FAA said three of those killed, including a patient, were aboard a Bell 407 helicopter operated by Air Methods Corp., an air medical service provider.

The other helicopter, operated by Classic Helicopter Service of Utah, also was a Bell 407. The other four victims, including one critically injured, were on that helicopter.

Flagstaff is about 120 kilometers south of the Grand Canyon National Park, which has seen a huge increase in helicopter use in recent years by visitors wanting air tours of the spectacular natural attractions. (dpa)