The November 2009 attack happened inside a crowded building where soldiers were waiting to get vaccines and routine paperwork after recently returning from deployments or preparing to go to Afghanistan and Iraq.
Army psychiatrist Nidal Hasan was convicted and sentenced to death last year in that mass shooting. He said he acted to protect Islamic insurgents abroad from American aggression.
According to testimony during Hasan's trial last August, Hasan walked inside carrying two weapons and several loaded magazines, shouted "Allahu Akbar!" — Arabic for "God is great!" — and opened fire with a handgun.
Witnesses said he targeted soldiers as he walked through the building, leaving pools of blood, spent casings and dying soldiers on the floor. Photos of the scene were shown to the 13 officers on the military jury.
The rampage ended when Hasan was shot in the back by Fort Hood police officers outside the building. He was paralyzed from the waist down and is now on death row at the military prison at Fort Leavenworth in Kansas.
After that shooting, the military tightened security at bases nationwide. Those measures included issuing security personnel long-barreled weapons, adding an insider-attack scenario to their training and strengthening ties to local law enforcement, according to Peter Daly, a vice admiral who retired from the Navy in 2011. The military also joined an FBI intelligence-sharing program aimed at identifying terror threats.
In September, a former Navy man opened fire at the Washington Navy Yard, leaving 13 people dead, including the gunman. After that shooting, Defense Secretary Chuck Hagel ordered the Pentagon to review security at all U.S. defense installations worldwide and examine the granting of security clearances that allow access to them.
Asked Wednesday about security improvements in the wake of other shootings at U.S. military bases, Hagel said, "Obviously when we have these kinds of tragedies on our bases, something's not working."
Associated Press writers Ramit Plushnick-Masti in Houston; Eric Tucker and Alicia Caldwell in Washington; Lolita C. Baldor in Honolulu; and Nedra Pickler in Chicago contributed to this report.
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