On September 16, 2013, a 34-year-old man goes on a rampage at the Navy Yard in Washington, D.C., killing 12 people and wounding several others over the course of an hour before he is fatally shot by police. Investigators later determined that the gunman, Aaron Alexis, a computer contractor for a private information technology firm, had acted alone.

Shortly after 8 a.m., Alexis used his security pass to enter Building 197 at the Navy Yard, a former shipyard, dating to the early 1800s, and weapons plant that now serves as an administrative center for the Navy. At approximately 8:16 a.m., Alexis, armed with a sawed-off Remington 870 shotgun and dressed in a short-sleeve polo shirt and pants, shot his first victim. Over the course of the next hour, he moved through the 630,000-square-foot, multi-level Building 197, the headquarters of the Naval Sea Systems Command, gunning down more victims and exchanging fire with law enforcement officials. Alexis was shot and killed by police at 9:25 a.m. The shooting spree caused officials to put part of Washington on lockdown due to initial suspicions that other gunmen might have been involved in the incident; however, by the end of the day, authorities determined that Alexis had acted alone.

A Navy reservist from 2007 to 2011, Alexis began work as a computer technician at the Navy Yard on September 9, 2013. Five days later, at a gun store in Virginia, he purchased the Remington 870 and ammunition used in the attack. Investigators found no evidence that any specific event triggered the deadly massacre, and they believed Alexis shot his victims at random. The shotgun he used (he also took a handgun from one of his victims) was etched with several phrases, including “Better off this way” and “My ELF weapon,” and the FBI announced there was a variety of evidence indicating Alexis was under the “delusional belief” he was being controlled by extremely low frequency (ELF) electromagnetic waves. In August 2013, Alexis told police in Rhode Island, where he was working, that he was hearing voices. The private IT contracting firm employing Alexis took him off his assignment for a few days then let him back on the job; weeks later, he went to work at the Navy Yard.

The 12 men and women murdered during the September 16th rampage ranged in age from 46 to 73. They were memorialized by then-President Barack Obama at a September 22, 2013, ceremony in which he remembered them and also issued a call to tighten America’s gun laws. That call largely went unheeded, and the number of mass shootings in the U.S. has continued to rise.