The gunman who killed nearly 60 people in a mass shooting in Las Vegas on Sunday had been prescribed anti-anxiety medications that can amplify aggressive behavior in individuals, according to a new report.

Stephen Paddock, 64, was prescribed 50 10-milligram tablets of diazepam, a strong sedative in a drug class called benzodiazepine that has dangerous side-effects, on June 21 by Dr. Steven Winkler, the Las Vegas Review-Journal learned. He was instructed to take one pill per day. 

Dr. Mel Pohl, the chief medical officer at the Las Vegas Recovery Center, told the Review-Journal that, like alcohol, people with underlying anger issues “can become aggressive” when sedated by the psychotropic drug, a finding confirmed by several medical studies. 

Diazepam can also cause hallucinations, “unusual risk-taking behavior” and “no fear of danger” in addition to hostility and aggression, according to drugs.com. 

Dr. Michael First, an expert on benzodiazepines who teaches psychiatric medicine at Columbia University, said the reason he was prescribed the drug in the first place “may have more to do with why he did what he did.” 

The prescribing doctor’s office would not tell the paper whether or not Paddock was a patient. 

Paddock killed at least 59 people and injured over 500 others in the shooting at a Las Vegas country music festival, making it the deadliest mass shooting in U.S. history.