GUANTANAMO BAY NAVAL BASE, Cuba — U.S. military tribunals at Guantanamo Bay resume this week even as new legal challenges could throw the system into further turmoil.

Five men charged in the Sept. 11 attacks, including alleged mastermind Khalid Sheikh Mohammed, are to appear today and Thursday for pretrial hearings in the Bush administration's special tribunal for terrorism suspects. Their trials have not yet been scheduled.

The suspects could get the death penalty if convicted of charges that include murder.

A judge is expected to hold hearings to explore defense allegations that Mohammed intimidated his co-defendants into refusing military lawyers.

Meanwhile, a judge in Washington is considering a challenge that could disrupt the first scheduled war crimes trial, on July 21, of Salim Hamdan, a former driver for Osama bin Laden.

Hamdan's lawyers say a recent Supreme Court decision has raised new legal issues that require U.S. District Judge James Robertson to delay the trial. The government says it wants to move forward.

Robertson has scheduled a July 17 hearing in Washington on the issue, just four days before Hamdan is to go on trial in a specially built courtroom on a former airstrip at the U.S. Navy base in Cuba.

A ruling in favor of the prisoner could also delay the trials for other men held at Guantanamo, and perhaps force the military to devise a whole new way to prosecute alleged terrorists.

"If a federal court enjoins the proceedings, that would be the death knell for the military commissions," said Sahr MuhammedAlly, a lawyer with Human Rights First.

Hamdan, a Yemeni, faces up to life in prison if convicted of supporting terrorism. The U.S. has said it plans to prosecute about 80 Guantanamo prisoners.