The witness, who asked not to be identified because it could damage his military career, has been interviewed by the Defense Criminal Investigative Division. He said Major Hasan wheeled on Sergeant Munley as she rounded the corner of a building and shot her, putting her on the ground. Then Major Hasan turned his back on her and started putting another magazine into his semiautomatic pistol.

It was at that moment that Senior Sgt. Mark Todd, a veteran police officer, rounded another corner of the building, found Major Hasan fumbling with his weapon and shot him.

How the authorities came to issue the original version of the story, which made Sergeant Munley a national hero for several days and obscured Sergeant Todd’s role, remains unclear. (Military officials also said for several hours after the shooting that Major Hasan had been killed, although he had survived.)