When the carnage ended, 13 were dead, including the shooter, and many others were injured.
Initially, authorities had indicated another male may have been involved, however, police now believe Aaron Alexis, 34, acted alone in the rampage.
USA Today reports that Alexis worked for an information technology subcontractor that updated equipment used by the Navy on its Intranet network.
An official who was not authorized to make a statement about the case told USA Today that Alexis purchased some of the weapons used in the shooting within the past several days in Virginia.
On Monday, Alexis drove to the Navy Yard, the official noted, and cleared security before parking his vehicle on the property.
The official also said he may have killed one or two people before entering the building and then headed to the third and fourth floors where most of the shooting took place.
CNN reports that Alexis enlisted in the Navy in 2007 and achieved a rank of Aviation Electrician’s Mate 3rd Class.
He served on the Fleet Logistics Support Squadron 46 at the Naval Air Station in Fort Worth, Texas.
Alexis was arrested in 2004 in Seattle for shooting out the tires of a construction worker’s car, believing he was being “mocked” and “disrespected,” USA Today reports. Later, he apparently told police he had been in New York on Sept. 11, 2001 and had participated in rescue efforts. His father told authorities he suffered from PTSD and anger management issues.
In 2010 in he was arrested in Fort Worth for discharging his gun within city limits, but was not charged.