On Monday, September 16, Aaron Alexis left his hotel and arrived at the Navy Yard in a rented Toyota Prius at around 7:53 a.m., using a valid pass to enter the Yard. He entered Building 197 at 8:08 a.m. through the main entrance, carrying the disassembled shotgun (the barrel and stock of which had been sawed off) in a bag on his shoulder, and went to the fourth floor, where he had conducted work during the prior week. He assembled the shotgun inside a bathroom, then emerged with the gun, crossed a hallway into the building’s 4 West area, which is a cubicle area near the atrium, and began shooting at 8:16 a.m. Four people were hit; three of them died, while the fourth, a young woman, survived wounds to the head and hand. At 8:17 a.m., approximately one minute and 30 seconds after the first shots were fired, the first calls to 9-1-1 were made. By 8:20 a.m., Alexis had shot and killed eight people on the fourth floor, and he made his way to the third floor, where he fatally shot two more people within the next two minutes. He also fired at several people on at least five separate occasions, wounding one woman in the shoulder as she ran up a stairwell. A NAVSEA employee described encountering a gunman wearing all-blue clothing in a third-floor hallway, saying, “He just turned and started firing.” After firing several shots on the third floor, Alexis went to the first floor.

At 8:23 a.m., officers from the D.C. Metropolitan Police Department and several other law enforcement agencies began arriving at Building 197. However, because there were many buildings on the base, officers were unable to discern Building 197’s location and asked bystanders for its location. They eventually found Building 197 after moving towards the direction people were fleeing from. There was also confusion regarding the shooting also taking place in a nearby building; in reality, a wounded victim evacuated from Building 197 had been moved to an area located near the second building for medical attention. While on the first floor, Aaron Alexis moved around randomly before turning around and heading towards the front entrance. There, he fired at Richard Ridgell, the security officer stationed there, through a set of windows, killing him and taking his Beretta 92FS 9mm semiautomatic pistol afterwards. Ridgell, a former Maryland state trooper, had earlier been informed by two police officers to remain at his post and try to stop the gunman if he attempted to leave the building. Alexis then fired his shotgun at a second security guard and a Navy military police officer at the first-floor atrium, missing both; the security guard fired back and Alexis fled down a hallway. Shortly afterwards, Alexis fired at two police officers and a Naval Criminal Investigative Service agent in another hallway before fleeing again.

At 8:34 a.m., Aaron Alexis went towards the west side of the building, where he encountered two men standing at a corner of the building in an alleyway. He tried to fire at them with his shotgun, but realized he was out of ammunition and switched to the stolen Beretta, shooting and killing one of the men with it; the other man managed to escape without injury. Reports indicated the victim in the alleyway was hit by a “stray bullet”. Alexis’s use of the pistol during the alleyway shooting led police officials to initially believe the possibility of a second gunman involved. After killing his final victim, Aaron Alexis moved to a cubicle area, where he discarded the shotgun. At the same time, a team of officers entered Building 197, but they became confused after gunshots echoed through the atrium, leading them to believe the gunman was on an upper floor. As a result, the officers headed up to the second floor, while Alexis remained on the first floor. At approximately 8:55 a.m., Alexis went to the third floor via stairwell and concealed himself inside a bank of cubicles. At 9:12 a.m., two officers and two NCIS agents entered the cubicle area, whereupon Alexis opened fire on them, hitting one of the officers, Scott Williams, in both legs. The other officer and the NCIS agents dragged Williams out of the area and alerted other officers to Alexis’ presence. Williams was later taken down to the first floor for medical attention, recovering from his wounds. At 9:15 a.m., soon after Williams’ evacuation, D.C. Police Emergency Response Team officer Dorian DeSantis and U.S. Park Police officers Andrew Wong and Carl Hiott entered the cubicle area and searched the individual banks. Eventually, Alexis jumped out from one of the desks and fired at DeSantis from approximately five feet away, shooting him once in his tactical vest, and the three officers returned fire. DeSantis was uninjured by the gunshot. At 9:25 a.m., Aaron Alexis was fatally shot in the head by DeSantis during the gunfight, and his death was later confirmed at 11:50 a.m.