The alleged gunman, Aaron Alexis, who killed a dozen people at the Washington Naval Yard on Monday was a former sailor who meditated regularly at a Buddhist temple in Texas to overcome his Post-Traumatic Stress Disorder (PTSD).
Alexis had been an active participant in rescue attempts on September 11, 2001 and had described to the police in 2004 how the events had disturbed him, News.com.au reports.
The 34-year-old is reported to have used guns in moments of anger, but was also known as a quiet man, the report added.
The defence contractor was killed in a gunfight with police that left several badly injured.
His father told detectives that Alexis had anger management problems associated with PTSD.
However, the police have not been able to speculate his motive behind the brutal attack.
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A U.S. military officer said there was a pattern of misconduct during his service from 2007 until 2011.
Alexis was also arrested in Seattle for shooting at the tyres of a car, which he described to be an anger-fueled blackout after construction workers had disrespected him.