Speaking at the Pentagon, Mr Obama paid tribute to the victims and said time would not diminish the loss.
He vowed to "never falter" in defending the US and pursuing al-Qaeda.
But the event was overshadowed by mistaken reports of a shooting, sparked by a Coast Guard training exercise on Washington's Potomac River. CNN had reported that 10 rounds were fired by the Coast Guard as a vessel tried to go through a security zone, after intercepting radio messages.
The Coast Guard later said in a statement that it was an exercise, and that it would conduct a thorough review to establish how the incident could have been perceived as real.
Presidential spokesman Robert Gibbs said that to his knowledge the White House had not been notified of the exercise but defended the Coast Guard's decision to hold it.
