Already, details of the sergeant’s identity and background as well as particulars of the judicial process he faces have been withheld longer than might be routine. Eugene R. Fidell, who teaches military justice at Yale Law School, said it was highly unusual for the sergeant’s identity to be concealed for long. Already, the deadlines for various stages of the due process that come with detention have passed or are approaching quickly, and once the formal pretrial Article 32 hearing is convened, his identity should be public.

A Pentagon spokesman, Capt. John Kirby, said in an interview that the suspect had been transferred to a United States military detention center outside Afghanistan, where he would remain in pretrial confinement while investigators from the Army Criminal Investigation Command continued their inquiry. Captain Kirby said the American military did not have a long-term detention center for service members in Afghanistan. A senior American official, confirming a report on CNN, said the sergeant had been taken to Kuwait.

Among the reasons for initially withholding the sergeant’s identity, Mr. Fidell said, may have been that the military wanted to protect his family from possible reprisal.

The Army sergeant’s surrender shortly after the killings was recorded on video taken at his military outpost in southern Afghanistan, according to an Afghan government official who saw the video. The video appears to back up American officials’ declaration that there was only one gunman.

Early Sunday morning, when military officials realized that the soldier was missing, commanders at the outpost started to scan the surrounding terrain using a video surveillance balloon, a device commonly used by foreign forces in Afghanistan to monitor insurgent activity. The black-and-white video shows the soldier walking by himself back to the outpost, Camp Belambi, in the Panjwai district in Kandahar. At that point, the soldiers at the base did not know about the massacre, the Afghan official said he had been told.