Experts say Johnson's medical path is the determining factor in comprehending his mindset on the fateful night of July 7, when he opened fire at a peaceful anti-police brutality rally.
According to the FBI, Johnson was armed with an SKS semiautomatic rifle and wore ballistic body armor with plates. The SKS is a Soviet-designed military rifle first introduced in 1945.
In March 2009, Johnson joined an Army Reserve unit in Seagoville, Texas, and served four and a half years. He was sent to Afghanistan in 2013 and served eight months with the 420th Engineer Brigade.
