OAKLAND -- The photo nearly speaks for itself. While the state continues its investigation into how a multi-million-dollar DC-10 outfitted to fight fires nearly crash in June, a photo given to KTVU shows the extent of the damage to the jet.

The DC-10 air tanker returned to duty in late July after being out of commission since June 25 when as it prepared to drop fire retardant along a Kern County ridge a severe downdraft forced the aircraft to sink and its left wing clipped several treetops.

"We're thankful that no one got hurt and the damage was as minimal as it was," said Rick Hatton, managing partner of the company that owns the DC-10. "It's a testament to the robustness of the airplane."

The converted plane was hailed by California fire officials last year as the first jumbo fire bomber of its kind to battle wildland fires. The U.S. Forest Service has refused to certify the plane over concerns about the number of hours already logged on the 31-year-old DC-10.

A preliminary report by the National Transportation Safety Board has revealed that the aircraft's left wing dropped downward while making a left turn. A state investigation was still ongoing.

While the cause of the accident has not been determined, the pilots say they hit air turbulence immediately before they hit the trees, according to Hatton.

Since the accident, two of the DC-10's captains and one co-pilot, all experienced airline pilots, have undergone refresher training for air-tanker operations and have been accompanying veteran tanker pilots in the state's medium-sized S-2T tankers, state fire aviation officials say.

The accident inflicted substantial damage on the leading and trailing edges of the jet's left wing, including portions of the leading-edge slats, the ailerons and flaps, according to the preliminary report and Hatton.

Fixing the plane amounted largely to removing and replacing the damaged parts. The No. 1 engine was unscathed.

"That was pretty lucky, because it hangs down off the left wing, and there was damage to either side of it," Hatton said.

Cal Fire air boss Mike Padilla says preliminary findings have determined that the cause of the near-crash was "mechanical."

"We're looking at everything from the weather -- turbulence -- on that date to our procedures for lining up the aircraft (toward the target) and how the pilots were perceiving the drop," Padilla said.