Investigators began piecing together on Friday how and why an Army psychiatrist facing deployment to Afghanistan gunned down 13 people a day earlier at the Fort Hood Army post in Texas, in one of the worst mass shootings ever on an American military base.

The gunman, identified as Major Nidal Malik Hasan, 39, was shot four times by a Fort Hood police officer responding to the scene. Maj Hasan remained hospitalised on a ventilator on Friday morning, but was stable, Army officials said.

Army officials said they had declared a day of mourning on the base for the 13 people killed — 12 soldiers and one civilian — and 28 wounded in the rampage.

Army officials said that about half the people injured in the shooting had undergone surgery, and all were stable. They praised the police officer who shot Major Hasan, Kimberly Munley, saying she and her partner had arrived within three minutes of reports of gunfire and put an end to the rampage.

But a day after the shooting unfolded, much still remained unknown about Maj Hasan’s life, motive and his forthcoming military assignment.

Clad in a military uniform and firing an automatic pistol and another weapon, Maj Hasan, a balding, chubby-faced man, sprayed bullets inside a crowded medical processing centre for soldiers returning from or about to be sent overseas, military officials said.

In an interview on NBC’s Today show, Lt Gen Robert W Cone, a base spokesman, was asked about reports that Maj Hasan had yelled “Allahu Akbar” — during the shooting.

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