SAN ANTONIO (AP) -- U.S. Air Force investigators have determined that post-traumatic stress disorder and the unraveling of a distinguished military career led an airman to fatally shoot his commander last year at a San Antonio base before killing himself, according to Air Force documents.

The April shooting at Joint Base San Antonio-Lackland prompted a lockdown and officials to abruptly end a nearby military training parade with thousands of spectators.

Investigators determined Tech. Sgt. Steven Bellino confronted Lt. Col. William Schroeder before the two struggled and Schroeder was shot multiple times. Both men were veterans of U.S. Special Operations Command.

Air Force documents given to the San Antonio Express-News (http://bit.ly/2jC5obt ) by Bellino's family show he participated in an elite pararescue program with Schroeder but did not complete it.

Investigators believe Bellino, 41, resented the outcome following a remarkable military career that included repeated tours in Afghanistan and Iraq and time as an Army Ranger and Green Beret. He also had served as an FBI agent and was a CIA contractor before enlisting in the Air Force and attempting to join the elite unit.

Friends say Bellino was idealistic and a man of exacting fairness, according to the newspaper. He lived up to the letter of the law and expected it of others, even once accusing a sergeant major of lying in front of a roomful of soldiers. But a series of perceived slights and violations of his sense of honor had accumulated long before he arrived at Lackland.

"I do not like this world, and I do not want to be a part of it any longer," Bellino wrote in August 2015, the month he quit the pararescue program and then went home to Ohio and was charged with being absent without leave. "I've searched for many years to find a home consistent with my ethics and such a place does not exist."

His comments came in a note that investigators found in a flash drive and they were written about the time his PTSD symptoms appeared to intensify.

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