Two Air Force pilots facing criminal charges over a mistaken bombing in Afghanistan have asked for a new investigation that they hope will clear them, their attorneys said Monday.

Attorneys for Majs. Harry Schmidt and William Umbach wrote to Air Force Secretary James Roche expressing concerns about the initial probe. The April 17 bombing killed four Canadian soldiers and injured eight.

"We asked the Secretary of the Air Force to look into how the investigation was done and that a new one be done," said Umbach's lawyer, Dave Beck, of Knoxville, Tenn.

He said the letter is not part of the formal legal process. "In a case like this, I don't think anything is 'normal,' but we set forth some concerns," said Beck, who declined to elaborate.

The letter suggests that Air Force Gen. T. Michael Moseley, who directed the investigation, had a conflict of interest, according to the State Journal-Register in Springfield, which obtained a copy for its Tuesday editions.

Moseley appointed a protege to conduct the probe, which concluded that "command and control failures" contributed to the accident, the letter said. The lawyers say Moseley was responsible for command and control.

The letter also said Moseley failed to include an Air National Guard member in the investigation, as required by Air Force regulations.

Schmidt and Umbach face charges of involuntary manslaughter, aggravated assault and dereliction of duty after Schmidt dropped a 500-pound bomb on a Canadian live-fire exercise. He says he thought the fire was hostile. Umbach was the mission's commander.

The airmen -- both members of the 170th Fighter Squadron of the Illinois Air National Guard based in Springfield -- face a hearing to determine whether the charges will be pursued.

Critics have complained that the government is making scapegoats of Schmidt and Umbach to maintain friendly relations with a staunch ally.