I am not sure "as many as two" is what they meant to present:
As many as two shooters, including one in fatigues, killed at least four people and wounded eight others in a rampage at the Washington Navy Yard on Monday, police said, spreading fear and chaos across the region as authorities tried to contain the incident.
Initial reports were marked by confusion, but by late morning, police said at least one of the shooters was “down.” It was unclear whether that means the suspect was in custody, wounded or dead. They said that another suspect may have been was pinned down in a building on the installation in Southeast Washington near Nationals Park.
"Another suspect" may have been pinned down? That suggests that perhaps no one was there, but also leaves open the possibility that one or more than one person was there.
WAPO UPDATE: Three shooters.
At least nine people are dead after as many as three shooters dressed in military style uniforms opened fire in a rampage at the Washington Navy Yard on Monday, police said, spreading fear and chaos across the region as authorities tried to contain the incident.
D.C. Police Chief Cathy Lanier said in a mid-day news conference that one of the suspected shooters is dead, while authorities are looking for two other suspects wearing green and tan military style clothing.