US President Donald Trump has ordered the execution of former US Army psychiatrist Major Nidal Hasan by firing squad, the Pentagon announced on Monday. Hasan was convicted for killing 13 people in a 2009 shooting at Fort Hood, Texas. The execution order marks the first time since 1961 that a former US service member has been sentenced to death under the US military justice system. Opened fire at Fort Hood in 2009 On November 5, 2009, Hasan arrived at Fort Hood carrying a privately owned pistol and opened fire on soldiers and other people at the military base. He shouted a religious slogan during the attack. The shooting was the deadliest mass killing at a US military installation. Hasan was also shot during the incident and was left paralysed from the waist down. At the time, the US was engaged in military operations in Afghanistan and Iraq. Hasan later said he considered those wars illegal and immoral and claimed he had carried out the attack in the name of jihad. Convicted of 45 counts of murder and attempted murder Hasan admitted to the killings during his military trial. In 2013, a military court convicted him on 45 counts, including premeditated murder and attempted murder. He is currently being held on military death row at Fort Leavenworth in Kansas. Hasan has shown no remorse for the attack. During proceedings before a military mental health panel, he said he wished he had also died during the shooting and described himself as a potential martyr. He also said that even if he were executed, he would still consider himself a martyr. Views became increasingly radical before attack Hasan was born in Arlington, Virginia, to Muslim parents of Palestinian origin. He studied at Virginia Tech before beginning medical training in the US Army in 1997. In the years leading up to the shooting, his views on Islam became increasingly radicalised. Pentagon spokesperson Sean Parnell referred to Hasan as a “convicted terrorist” while describing the jihadist motive behind the attack. However, Hasan was not prosecuted under terrorism-related charges. The US Army had deliberately chosen not to bring terrorism-related charges against him.