SPRINGFIELD, Ill. -- The Air Force pilots facing criminal charges over a mistaken bombing in Afghanistan have asked for a new investigation, their attorneys said Monday.

Attorneys for Majs. Harry Schmidt of Sherman and William Umbach of Petersburg wrote to Air Force Secretary James Roche expressing concerns about the investigation. 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, although both he and Schmidt's lawyer, Charles Gittins of Middletown, Va., have written such letters in past cases.

"In a case like this, I don't think anything is 'normal,' but we set forth some concerns," said Beck, who declined to elaborate.

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 that he says he thought was hostile. Umbach was the mission's commander.

Both are members of the 170th Fighter Squadron of the Illinois Air National Guard based in Springfield.

Eighth Air Force Commander Lt. Bruce Carlson has decided the two will face a hearing to determine whether the criminal charges will be pursued.

Critics have complained that the government is making scapegoats of Schmidt and Umbach to keep relations friendly with a staunch ally. They say the pair were not told of "friendlies" in the area.

"I would expect that the secretary, if he is truly interested in the welfare and morale of the Air Force, will give thoughtful consideration to our request," Gittins said in an e-mail to The Associated Press.