“Some of the witnesses that experienced this tragic event are still seeking medical attention, and we have not been able to talk to them,” he said, flanked by lead investigators from the Army, the F.B.I. and the Texas Rangers.

Mr. Grey said a motive for the shootings had yet to be determined. He also said investigators believed that Major Hasan had been the only gunman at the scene, had had no orders from Army superiors to be there and had had no scheduled appointments there.

Major Hasan was supposed to deploy to Afghanistan on Nov. 28 with several Army Reserve units of mental health professionals trained to deal with combat stress. Five of the soldiers he is charged with killing came from those Reserve units. The civilian who died, Michael Grant Cahill, 62, was a physician’s assistant who had been working at Fort Hood and, according to his family, had also worked with soldiers dealing with mental health problems who were either going or returning from overseas.

President Obama has ordered all government agencies to search their files for any information obtained about Major Hasan before the shooting, the White House announced Thursday. In the order, dated Tuesday, the president also directed the F.B.I., the Defense Department and intelligence agencies to assess how the information had been “handled, shared and acted upon.”

Mr. Grey took no questions at the news conference about the precise circumstances in which Major Hasan was brought down by the Army’s civilian police.

“Our investigation thus far,” he said in his statement, “indicates that two responding police officers, one male and one female, arrived at the scene and both engaged the armed suspect. I would caution anyone from drawing final conclusions concerning the actual engagement, in terms of who did what, until all the evidence is fully analyzed.”

Military officials initially reported that one police officer, Sgt. Kimberly D. Munley, had wounded the gunman during an exchange of gunfire with him. But a second civilian officer, Senior Sgt. Mark Todd, said in an interview on Thursday that he shot Major Hasan while Sergeant Munley, already wounded, was lying on the ground. He said he did not know whether Sergeant Munley too had shot the major.