The U.S. Air Force has rejected a bid by two American fighter pilots for a new investigation into the accidental bombing that killed four Canadian soldiers in April.

The request was denied after lawyers for pilots Maj. Harry Schmidt and Maj. Bill Umbach argued that the original investigation that led to the criminal charges did not follow proper air force regulations.

Both men face manslaughter charges in the bombing, which occurred during a live-fire exercise in Afghanistan. Eight Canadian soldiers were also wounded in the attack.

Lawyers had petitioned Secretary of the Air Force James Roche to reconsider the investigation, saying the panel which recommended charges did not consist of a member of the Air National Guard.

But Col. Craig Smith, an air force lawyer, concluded that the probe was conducted by the Department of Defence, not the air force.

"Therefore, air force instructions on how to conduct such an investigation didn't govern this investigation," spokeswoman Valerie Burke told the St. Louis Post-Dispatch on Tuesday.

The men must now attend a hearing in January for a so-called Article 32, which will then determine if the men should be court martialled.

If convicted, they could be sentenced to 64 years in prison, be dishonorably discharged and lose all pay and allowances.

Schmidt, 37, is a combat-decorated navy pilot who transferred to the National Guard last year. Umbach, 43, is a United Airlines pilot who had served in the air force. Both men were called up for service following Sept. 11.

A Canadian and joint Canada-U.S. military inquiry accuses the pilots of not following procedure before dropping the bomb.

Schmidt, who actually dropped the bomb, told investigators he thought the fire was hostile and dropped the 226-kilogram bomb in self-defence.

The charges also include allegations of poor airmanship, poor judgment and failure to follow procedures, such as flying higher to avoid ground fire until more information was gathered.

Evidence indicates the pilots were not briefed about the training exercise and that there were conflicting messages about whether they could fire.

Charles Gittins, the lawyer for Schmidt, claims the investigation was biased because it ignored command-and-control failures.

Gittins told CTV News recently that he believes the pilots are unfairly being singled out while senior U.S. military officials evade responsibility for the tragedy.

"We are very concerned that the pilots have been the only ones who have been charged in this matter," Gittins told CTV last month.

Neither Schmidt nor Umbach have publicly commented on the charges against them, but their family members have pleaded for leniency.