Las Vegas killer Stephen Paddock was prescribed the anti-anxiety drug Valium in June, a drug that has aggressive behavior as a possible side effect.
The Las Vegas Review-Journal reported Paddock was prescribed the medication in June. He was supposed to take one pill per day and fulfilled the prescription on the same day it was written.
"If somebody has an underlying aggression problem and you sedate them with that drug, they can become aggressive," said Dr. Mel Pohl, chief medical officer of the Las Vegas Recovery Center, told the newspaper. "It can disinhibit an underlying emotional state. ... It is much like what happens when you give alcohol to some people ... they become aggressive instead of going to sleep."
Paddock killed 59 people and injured more than 500 others when he opened fire with high-powered rifles from the 32nd floor of the Mandalay Bay Resort and Hotel late Sunday night. He shot into a country music festival taking place on the street below.
Officials continue to investigate the incident, the largest mass shooting in American history.
Questions remain over whey Paddock wired $100,000 to the Philippines just before the shooting. The island nation is the home country of his girlfriend, who was out of the country at the time of the shooting.
He also reportedly gambled with more than $10,000 during the day before the shooting.