President Barack Obama ordered American flags outside the White House to be lowered to half staff immediately, to honor at least 12 victims of the Washington Navy Yard shooting today.

The American flags will be lowered to half staff until sunset on Friday, in honor of the Washington Navy Yard victims, The Huffington Post reports.

The White House has also postponed a Musica Latina concert originally slated for Monday evening to a later date. The president and first lady were both scheduled to attend the concert, the website notes.

Obama spoke at an event marking the fifth anniversary of the financial crisis, but turned his attention to the shooting and said: "We are confronting yet another mass shooting, and today it happened on a military installation in our nation's capital."

"It's a shooting that targeted our military and civilian personnel," he said, NBC News reports. "These are men and women who were going to work, doing their job, protecting all of us. They're patriots. And they know the dangers of serving abroad, but today they faced the unimaginable violence that they wouldn't have expected here at home."

According to NBC News, a former naval petty officer carrying an assault rifle and two other guns fired shots in an office building at the Washington Navy Yard on Monday, killing 12 people.

It was the deadliest mass shooting in the United States since the massacre at Sandy Hook Elementary School last year.

Authorities said the gunman has been identified as a civilian contractor named Aaron Alexis, 34. Alexis was killed after exchanging fire with officers who stormed the headquarters of the Naval Sea Systems Command in a massive law enforcement response, NBC News writes.

Authorities told a late-afternoon press conference that they were still looking for one man considered a possible suspect.