Initial reports were that the two soldiers – part of a militarised anti-crime deployment around the US ordered by Trump – had been badly wounded.
West Virginia governor Patrick Morrisey said the soldiers, both deployed to the capital from his state, had “passed away from their injuries,” but then said there were “conflicting reports.”
It was the most serious incident involving the national guard since Trump began ordering troops onto the streets of several Democratic-run cities shortly after starting his second term this January.
The Republican, who was at his Florida golf club, described the shooter as an “animal”.
He said the two soldiers were “critically wounded”.
The suspect “is also severely wounded, but regardless, will pay a very steep price,” Trump posted on his Truth Social platform.
An AFP reporter near the scene heard several loud pops and saw people running.
Dozens of bystanders were caught up in the chaos.
“We heard gunshots. We were waiting at the traffic light and there were several shots,” said Angela Perry, 42, who was driving home with her two children.
“You could see national guard running toward the metro with their weapons drawn.”
Soon after the shootings, security agents flooded the area around the Farragut West metro station, two blocks from the White House. Officers carrying rifles stood guard behind yellow tape at the perimeter of the area and a helicopter circled over the busy downtown.
An AFP reporter saw emergency crews running toward the metro with a wheeled stretcher and shortly after emerging with a casualty wearing camouflage, whom they loaded into an ambulance.
The government buildings of downtown Washington are heavily guarded, but much of the city has suffered from years of sometimes serious street crime.
Trump made Washington a showcase for his decision to order national guard soldiers, in camouflage and occasionally carrying rifles, to patrol streets of cities run by Democratic mayors, also including Los Angeles and Memphis.
The policy has prompted bitter complaint from local officials who accuse Trump of stoking tensions, while the deployments are the subject of numerous court challenges.
Last Thursday a federal judge ruled that Trump’s deployment of thousands of national guard troops in the US capital is unlawful.