(Modena, UT) - A heavy air tanker that crashed while dropping fire retardant on a Utah wildfire last week veered from the path of the spotter plane in front of it before slamming into a mountainside, according to a preliminary report issued Tuesday on the accident by the National Transportation Board.

The deaths of the air tanker's two-man crew, pilot Todd Tompkins, 48, and co-pilot Ronnie Chambless, 40, both of Boise, Idaho, marked the year's first fatalities among personnel fighting US wildfires. A memorial service for air tanker pilots will be held Thursday, June 14th at 7:00PM in the Linen Building at 1402 West Grove Street in Boise. For addition information, go to Memorial Service

The Utah air tanker owned by Neptune Aviation Services of Missoula, Montana, a Lockheed Martin P2V, was flying under contract to the US Forest Service. The plane crashed June 3 on a forested mountain in the Hamlin Valley area of southwestern Utah while battling the White Rock Fire along the Utah-Nevada border.

A two-paragraph initial report on the crash posted on the website of the National Transportation Safety Board does not rule out mechanical failures or other possible causes for the accident.

It also does not indicate whether smoke from the blaze, which had burned from Nevada into Utah, caused visibility problems for the pilots. The pilots worked for Neptune Aviation company, based in Missoula, Montana, and had taken off that day from Cedar City, Utah.

NTSB spokesman Keith Holloway said, "At this point we have not ruled out much of anything. We are just documenting what we know."

The air tanker, designated Tanker 11, had taken off 32 minutes before the crash and was trailing a smaller lead plane over a fire-retardant drop zone in a valley just under a half-mile wide and 350 feet deep, the report said. A lead plane is used to guide air tankers in the suppression of wildfires.

The spotter aircraft dipped to an altitude of 150 feet above the valley floor and made a shallow right turn toward the final drop area, the report said.

"While making the right turn on to final (approach) behind the lead plane, Tanker 11 impacted rising terrain that was about 700 feet left of the lead airplane's flight path," according to the report.

The plane's flight recorder was recovered by investigators, and its data will be analyzed along with pieces of the aircraft removed from the crash site, Holloway said. A final report is expected within 18 months.