RENO, Nev. — Federal safety regulators are focusing on loose screws in the tail of a World War II-era fighter plane modified to race faster than 500 mph as a likely cause of the horrific crash that killed the pilot and 10 others during air races at the Reno National Championship Air Races last September.

The National Transportation Safety Board released nearly 1,000 pages of documents and photographs on Tuesday while continuing to investigate the official cause of the accident that also injured more than 70 people at Reno Stead Airport.

Among other things, the documents point to potential deficiencies in the safety inspection procedures, something race organizers said they're addressing at this year's 49th annual National Championship Air Races, scheduled for Sept. 12-16.

One NTSB document shows inspectors noted about a month before the race that the screws were too short in one of the trim tabs that helped control the P-51 Mustang flown by Jimmy Leeward.

According to interviews with the technical inspectors and the crew, the trim tab problem "was due to one or more screws on the right elevator trim tab not having enough threads protruding from the nut and there was an area washer missing in the wheel well."

The crew reportedly fixed the problems and the plane was cleared to race, but the NTSB said there was "no written procedure or sign off to ensure" that was the case.

Formalizing plane inspection procedures was one of the recommendations of a panel of experts air race officials commissioned in the aftermath of the crash to help make the event safer.

Specifically, the panel led by former NTSB chairman Jim Hall said in May it is important to be sure "uncorrected discrepancies" regarding airplane modifications "do not slip through the system."

The competition about eight miles north of Reno is the only event of its kind in the world, with planes flying wing-tip-to-wing tip around an oval pylon track, sometimes just 50 feet off the ground and at speeds of over 500 mph.

Leeward, 74, was traveling at 530 mph when his plane dubbed the "Galloping Ghost" pitched skyward while making a turn, then rolled and slammed into the ground nose first near box seats.

Still photos showed part of the tail known as the elevator trim tab came off as the plane went out of control and aviation experts theorized at the time it could have caused the crash.

The NTSB documents show investigators also are looking closely at the possibility turbulence from a plane ahead contributed to Leeward's loss of control but don't think that alone would cause the accident.