Two National Guard soldiers have been shot near the White House in what US authorities say was a targeted ambush.
US President Donald Trump said the attack was an "act of terror" and the suspect was from Afghanistan.
Both soldiers were critically injured, police said, after a state governor earlier tweeted they had died.
The soldiers, who were on patrol in Washington as part of a Trump-ordered crime crackdown, were shot about at 2:15pm on Wednesday, local time.
Investigators identified the suspect as Rahmanullah Lakanwal, a 29-year-old Afghan national from Washington State, according to a Justice Department official who spoke on condition of anonymity to Reuters and AP.
The attack was being investigated as an act of terrorism, the official said.
Mr Lakanwal came to the US in 2021 on a special visa program for Afghans who assisted the US during the Afghanistan war and were vulnerable to reprisals from the ruling Taliban.
He overstayed his visa and was in the country illegally, according to the official.
Mr Trump said the US would now "re-examine" everyone who arrived from Afghanistan under the previous Biden administration.
"Based on the best available information, Department of Homeland Security is confident that the suspect in custody is a foreigner who entered the country from Afghanistan, a hellhole on Earth, flown in by the Biden administration on September 2021," he said.
"It was an act of evil, an act of hatred and an act of terror. It was a crime against our entire nation," he said.
He added said this was a "monstrous, ambush-style attack".
Suspect allegedly 'ambushed' the guards
"A suspect came around the corner, raised his arm with a firearm and discharged it at the National Guard," local police executive assistant chief Jeffery Carroll said.
The suspect was shot before he was detained, Chief Carroll said, but it was not clear who shot him. There were no other suspects, he said.
"It appears, like I said, to be a lone gunman that raised the firearm and ambushed these members of the National Guard, and he was quickly taken into custody by other National Guard members and law enforcement members."
Mayor Muriel Bowser said it was a "targeted shooting". "One individual who appeared to target these guardsmen," she said.
But Chief Carroll said the shooter's motive was not yet known.
Police and emergency crews cordoned off an area of downtown Washington, where National Guard members from multiple states have been on patrol since August.
Many of Washington's government workers had finished work early ahead of Thursday's Thanksgiving holiday.
The deployment of armed soldiers on Washington's streets has been controversial in the US capital, where police data has shown crime had been declining before Mr Trump ordered the crackdown.
Mr Trump has now ordered another 500 National Guard members to deploy in the city, Defense Secretary Pete Hegseth said.
"This will only stiffen our resolve to ensure that we make Washington DC safe and beautiful," he said.
Earlier, West Virginia Governor Patrick Morrisey wrote on social media that both soldiers were from his state and both had died. But he later said there were "conflicting reports" about their conditions.
Mr Morrisey initially wrote: "It is with great sorrow that we can confirm both members of the West Virginia National Guard who were shot earlier today in Washington, DC have passed away from their injuries."
But about 20 minutes later, he posted: "We are now receiving conflicting reports about the condition of our two Guard members and will provide additional updates once we receive more complete information."
