Even if he had no doubt about the guilt of the accused, he wrote in an August e-mail, "I am beginning to have grave misgivings about what I am doing, and what we are doing as a country. . . .

"I no longer want to participate in the system, but I lack the courage to quit. I am married, with children, and not only will they suffer, I'll lose a lot of friends."

Two days later, he took the unusual step of reaching out for advice from his opposing counsel, a military defense lawyer.

"How do I get myself out of this office?" Vandeveld asked Major David J.R. Frakt of the Air Force Reserve, who represented the young Afghan Vandeveld was prosecuting for an attack on U.S. soldiers -- despite Vandeveld's doubts about whether Mohammed Jawad would get a fair trial. Vandeveld said he was seeking a "practical way of extricating myself from this mess."

Last month, Vandeveld did just that, resigning from the Jawad case, the military commissions overall and, ultimately, active military duty. In doing so, he has become even more of a central figure in the "mess" he considers Guantanamo to be.

Vandeveld is at least the fourth prosecutor to resign under protest. Questions about the fairness of the tribunals have been raised by the very people charged with conducting them, according to legal experts, human rights observers and current and former military officials.

Vandeveld went to Guantanamo, where he began locking horns over the Jawad case with Frakt -- a law professor at Western State University in Fullerton and a former active-duty Air Force lawyer who volunteered for the tribunals.

Frakt believed that his Afghan client was, at worst, a confused teen who had been brainwashed and drugged by militant extremists who coerced him into participating in a grenade-throwing incident with other older -- and more guilty -- men. He insisted that the prosecution was withholding key information or not obtaining it from those at the Pentagon, CIA and other U.S. agencies that had investigated and interrogated Jawad.

Vandeveld believed that Jawad was a war criminal who had been taught by an Al Qaeda-linked group to kill American troops and, if caught, to make up claims he had been tortured and was underage. Vandeveld insisted that he had been providing all evidence to the defense.

But by July, Vandeveld told The Times, he had grown increasingly troubled. He kept finding sources of information and documents that appeared to bolster Frakt's claims that evidence was being withheld -- including some favorable to the defense, such as information suggesting that Jawad was underage, that he had been drugged before the incident and that he had been abused by U.S. forces afterward.

Vandeveld also was having difficulty obtaining authorization to release documents in his possession to the defense.