The alleged shooter, Aaron Alexis, 34, of Fort Worth, Texas, was a full-time member of the U.S. Naval Reserve from 2007 to January 2011, according to a Navy spokeswoman. Alexis, who was born in Queens, N.Y., was confirmed to be one of the dead at the scene, authorities said.

Alexis worked at the Navy Yard in information technology for a company called The Experts, a subcontractor for HP Enterprise Services. The 34-year-old had no apparent misconduct issues at The Experts, CEO Thomas Hoshko said. But Alexis was treated by the U.S. government for serious mental illness, souces told The Associated Press Tuesday morning.

Hoshko added that it is unclear if Alexis had just started working at the Navy Yard or was just about to start. The FBI said that Alexis had entered the Navy Yard with a valid pass.

Earlier Monday, authorities were looking for two additional people of interest, but Washington Police Chief Cathy Lanier said Monday night about Alexis that "we are comfortable that we have the single and sole person responsible."

Shots reportedly first rang out at 8:20 a.m. in Building 197, headquarters of the Naval Sea Systems Command, which is about 1.5 miles southeast of the U.S. Capitol and the National Mall.

Three victims were admitted to Washington Hospital Center in critical condition. Two required surgery, but all three of the wounded are doing well, hospital spokeswoman Janis Orlowski said. One of injured included Scott Williams, a 23-year veteran of the D.C. police.

The deceased victims were inside the Navy headquarters, according to Lanier. Mayor Gray described the event as an "isolated incident" and said that no other facilities in the city were involved.

Speaking at a press conference, President Barack Obama said he is going to do "everything in our power to ensure that whoever carried out this cowardly act is held responsible."

Monday's attack has resulted in the most fatalities in a mass shooting this year, and the deadliest single-day event in Washington D.C. since a 1982 plane crash that killed 78 people.

So far, police have confirmed the names and ages of seven of the 13 victims killed in the shooting whose families have been notified. The victims include Michael Arnold, 59, Sylvia Frasier, 53, Kathy Gaarde, 62, John Roger Johnson, 73, Frank Kohler, 50, Bernard Proctor, 46 and Vishnu Pandit, 61.

TIMELINE: U.S. massacres tripled over past decade

The Washington Navy Yard, located in southeast Washington, D.C., along the Anacostia River, is the nation's oldest on-shore naval facility. Three thousand people work at the Naval headquarters.

As the shooting investigation continues, only essential personnel are to report back to work at the Navy Yard Tuesday.

LIVE BLOG: Navy Yard shooting