Thirteen are dead—including the attacker—and at least eight wounded after a gunman opened fire Monday morning at the Washington Navy Yard in Washington, DC. At least three victims were hoisted from Naval Sea Systems Command Headquarters Building 197 by a National Park Police helicopter, and the District of Columbia Metropolitan Police Department (MPD) reports that a police officer was seriously injured in the shootout but is expected to survive.

Despite many earlier reports about multiple assailants, including detailed descriptions of other suspects, it now appears that a single man—a contractor named Aaron Alexis, 34, from Fort Worth, Texas—was responsible for the carnage. Officials believe that he arrived at the Navy Yard armed with a shotgun, and later took two handguns from law enforcement officers on-scene. Earlier reports claimed that Alexis also obtained an AR-15-style rifle during the attack, which likely would have been a semi-automatic model (not an ‘assault rifle,’ as it has been labeled by many media outlets), but those reports were in error. Only the shotgun and two handguns were found at the scene.

Alexis was a former full-time Navy Reservist who was honorably discharged in 2011 following a number of disciplinary problems. He had suffered from a variety of serious mental health problems and had sought treatment, and yet retained a security clearance and had the appropriate access rights to enter the Navy Yard property. Alexis purchased the shotgun used in the assault less than one week ago in Virginia, after having been cleared by the the National Instant Criminal Background Check System (NICS). It is unclear why somebody with documented mental health problems both cleared the NICS check and was granted access to a secured military facility.

As events unfolded on Monday, nearby Reagan National Airport was briefly closed. The Eleventh Street bridge was also closed for much of the day, and nearby schools were put on a day-long lock-down before dismissing at the usual time. The United States Coast Guard also established a secure zone along the Anacostia River shore and briefly stopped all Anacostia and Potomac river traffic. There were reports of another shooting incident at Joint Base Anacostia-Bolling, located further south on the Anacostia and Potomac rivers, but those reports were in error.