An unborn baby was among those who died when Devin Patrick Kelley stormed the First Baptist Church in Sutherland Springs last November and opened fire.
Medical tests, first reported on by the San Antonio-based broadcaster KENS and later by Associated Press, also concluded that small levels of cannabis and anti-anxiety drugs were detected in his system.
The results of an examination on the 26-year-old's body were released by the Travis County Medical Examiner's Office.
Kelley was found dead inside a vehicle after being shot and chased by two men who heard him shooting worshippers on 5th November.
The 14-year-old daughter of the church's pastor Frank Pomeroy was among several children who died in the incident which is the deadliest mass attack Texas has ever seen.
According to investigators, it may have been linked to a rift between Kelley and his mother-in-law, who attended the church but was not there on the day of the shooting.
He had previously been thrown out of the US Air Force for assaulting his spouse and child.
One family which lost several relatives in the shooting is suing the federal government, claiming that failings by the military allowed Kelley to purchase the rifle he used in the shooting.
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