Two West Virginia National Guard members were shot in downtown Washington, just blocks away from the White House, in a brazen act of violence.
Police have taken one person into custody in connection with the incident, which Mayor Muriel Bowser called a "targeted shooting".
National Guard troops from multiple states have been in Washington, DC, for months as part of President Donald Trump's anti-crime crackdown in the nation's capital.
Condemning the incident, Trump wrote on X that the culprit will have to pay a heavy price.
"The animal that shot the two National Guardsmen, with both being critically wounded, and now in two separate hospitals, is also severely wounded, but regardless, will pay a very steep price," he said.
"God bless our Great National Guard, and all of our Military and Law Enforcement. These are truly Great People. I, as President of the United States, and everyone associated with the Office of the Presidency, am with you!" Trump added in his post.
Following the shooting, the Trump administration quickly ordered 500 more National Guard members to Washington.
Earlier, addressing a news conference, FBI Director Kash Patel and Mayor Muriel Bowser said the Guard members were hospitalised in critical condition.
"Since this is an assault on a federal law enforcement officer, this will be treated at the federal level as an assault on a federal law enforcement officer," Patel said.
"The FBI will lead out on that mission with our interagency partners to include the Department of Homeland Security, Secret Service, ATF, DEA, and we're thankful for the mayor's assistance in this matter," he said.
Jeffrey Carroll, an executive assistant to the DC police chief, said the assailant "came around the corner" and immediately started firing at the troops.
The presence of the National Guard in Washington, DC had triggered a public policy debate about the Trump administration's use of the military to combat what officials cast as an out-of-control crime problem, according to Associated Press.
Over 300 West Virginia National Guard members were deployed to Washington in August. Last week, about 160 of them volunteered to extend their deployment until the end of the year while the others returned to West Virginia.