GUANTANAMO BAY NAVAL BASE, Cuba—Khalid Sheikh Mohammed, the reputed mastermind of the Sept. 11 attacks, told a military judge at his arraignment Thursday that he welcomes the death penalty as a way to martyrdom and ridiculed the proceedings as an “inquisition.”

In his first public appearance since his capture five years ago, Mohammed wore thick glasses, a turban and a bushy, gray beard, and he was noticeably thinner—a stark change from the slovenly man with disheveled hair, unshaven face and T-shirt from the widely distributed photograph after his seizure in Pakistan.

He and four other detainees accused of plotting al-Qaida’s 2001 attack were at turns cordial and defiant at their arraignment, the first U.S. attempt to try in court those believed to be directly responsible for killing 2,973 people in the bloodiest terrorist attack ever on U.S. soil.

The arraignment is the highest-profile test yet of the military’s tribunal system, which faces an uncertain future at Guantanamo. It also could expose harsh interrogation techniques used on the men, who were in CIA custody before being transferred to Guantanamo in 2006.

The government is seeking the death penalty for all five defendants, who sat at separate tables with their defense teams in a high-tech courtroom on this U.S. Navy base.