The Pentagon official in charge of war crimes cases declined to permit a case to proceed against one of six detainees charged in the Sept. 11, 2001, attacks, dismissing charges against a Saudi who had been subjected to aggressive interrogation at the United States detention camp at Guantánamo Bay, Cuba.

The decision by the official, Susan J. Crawford, whose title is Convening Authority, came with no explanation.

She approved the death penalty charges submitted by military commission prosecutors in February against five detainees in the 2001 attacks, while declining to approve charges against the sixth detainee, Mohammed al-Qahtani. Mr. Qahtani was subjected to interrogations that Pentagon officials have found were “degrading and abusive,” including being forced to wear a bra, being led around on a leash and required to perform dog tricks.

Ms. Crawford’s decision said the charges against Mr. Qahtani were being dismissed “without prejudice.” On Tuesday, Capt. André Kok of the Air Force, a spokesman for her and for military prosecutors, said the government could “reinitiate charges against him at any time.”

The chief prosecutor at Guantánamo, Col. Lawrence J. Morris of the Army, said, through Captain Kok, that prosecutors were “evaluating the case in light of the decision to sever.”

In legal parlance, severance of a case can mean that it could proceed separately.

Mr. Qahtani has sometimes been referred to as the 20th hijacker because of evidence that he tried to enter the United States a few days before Sept. 11, 2001, and that he was in touch with the men who became the hijackers.

Mr. Qahtani’s military lawyer, Lt. Col. Bryan Broyles of the Army, said he believed that Ms. Crawford’s decision meant that efforts to try Mr. Qahtani were over. “It is unlikely that they are ever going to try him,” Colonel Broyles said.

He said he believed Ms. Crawford might have concluded that evidence against Mr. Qahtani was “derived by torture,” which could undermine the prosecution’s case.

The decision permitted the prosecutors to proceed with the case against Khalid Shaikh Mohammed, the self-professed planner of the attacks, and four other men held at Guantánamo whom officials describe as “high-value detainees.” The five were held in secret C.I.A. prisons until they were transferred to Guantánamo in 2006.

Mr. Mohammed’s lawyer, Capt. Prescott L. Prince of the Navy, said the effort to press ahead with the charges against those detainees was political. He said prosecutors were trying to rely on evidence obtained through coercion or torture that would not be admissible in civilian or regular military courts.

Mr. Mohammed is one of three former C.I.A. prisoners government officials have said were questioned using the simulated drowning technique known as waterboarding.

Military lawyers for several of the five detainees said they expected to begin extensive legal challenges to the military commission system. “Given the political nature of the entire process,” Captain Prince said, “I just don’t see how my client can get a fair trial before military commissions.”

Lawyers said it was difficult to analyze the significance of Ms. Crawford’s decision, particularly since the charges were originally announced in February in a news conference by her legal adviser, Brig. Gen. Thomas W. Hartmann.

Questioned about the decision, the spokesman, Captain Kok, said commission rules “do not require the Convening Authority to explain her decisions.”

Ms. Crawford is a quasi-judicial official with broad powers, including the power to reduce or reject charges brought by prosecutors.

But lawyers involved with the defense said they had seen a memorandum to Ms. Crawford from General Hartmann that could help explain the decision. They said General Hartmann had concluded that seeking the death penalty was appropriate for Mr. Mohammed and the other four detainees, who have been described as high-level planners and facilitators.

But the lawyers said General Hartmann concluded that prosecutors should not seek the death penalty for Mr. Qahtani. One lawyer who saw the memorandum said it did not include a reason why.

But other lawyers noted that Mr. Qahtani might have been only a low-level plotter, and an unsuccessful one at that.

Other lawyers said a factor might have been the long record of Mr. Qahtani’s treatment at Guantánamo. Gitanjali Gutierrez, a lawyer with the Center for Constitutional Rights who is representing Mr. Qahtani, said it appeared that officials were pressing for quick trials and that the issues of Mr. Qahtani’s treatment might have been an obstacle. “There’s no question about what happened to him,” she said.

The timing of the announcement was unexpected, coming days after a military judge in another case disqualified General Hartmann from participating in that case, saying the general was so closely aligned with the prosecution that it was not clear he could carry out his role with the required neutrality. That case is against a detainee who was a driver for Osama bin Laden.

On Tuesday, several of the military defense lawyers said they would seek dismissal of the charges against the five detainees in the Sept. 11 case because of General Hartmann’s involvement. Several lawyers said they were perplexed about why Ms. Crawford would proceed when it was clear that General Hartmann’s participation would bring new legal challenges.

Pentagon spokesmen declined to comment.

But Maj. Jon S. Jackson, the defense lawyer for one of the men charged, Mustafa Ahmad al-Hawsawi, said the decision to proceed despite the general’s involvement was “illegal because he has acted outside his role as a neutral and detached legal adviser.”

Copyright 2008 The New York Times Company