One of those witnesses, an Afghan militiaman, corroborated the American soldiers’ accounts that the detainees were assaulted by Afghan and American personnel, according to an N.C.I.S. report written in August 2012. A shopkeeper, the only one of the four witnesses who saw the dead body, told the N.C.I.S. that he was beaten by the Afghan militiamen but not by Americans.

Still, several former military lawyers told The Times that the sworn statements by the soldiers had provided more than enough evidence for the SEAL command to call for a hearing before an impartial Navy lawyer, who could have decided whether to recommend a court-martial on assault or even manslaughter charges.

“My reaction is better late than never,” Eugene R. Fidell, who teaches military justice at Yale Law School, said of the N.C.I.S. decision to reopen the investigation. “I think the command’s dismissive attitude toward evidence presented by members of another branch of the armed services was disturbing.”

Former military lawyers cautioned that it could be difficult for the N.C.I.S. to locate the new witnesses and persuade them to talk to the American authorities. The episode happened near the remote village of Kalach in Oruzgan Province, and the area is now back in Taliban control. It took months for The Times to locate the men and arrange to meet them in Kabul for interviews.

The detainee who said he saw the other prisoner die goes, like many Afghans, by one name, Assadullah. He told The Times that he, another companion, and the 24-year-old man who died, Muhammad Hashem, were itinerant scrap merchants who were rounded up by the Afghan Local Police after a roadside bomb killed a militiaman near Kalach.

He said the militiamen took the three of them and several villagers from Kalach to the American outpost for questioning and that American personnel had grabbed him by the throat and kneed him in the stomach several times.