The suspect had worked in a special CIA-backed Afghan army unit before emigrating from Afghanistan, according to a cousin who spoke to the Associated Press on condition of anonymity for fear of reprisals.
Jeanine Pirro, the US attorney for the District of Columbia, declined to provide a motive for Wednesday afternoon's brazen act of violence.
Pirro identified the guard members at a news conference as Specialist Sarah Beckstrom, 20, and Staff Sergeant Andrew Wolfe, 24.
Both remained hospitalised in critical condition.
Pirro said that the suspect, Rahmanullah Lakanwal, launched an "ambush-style" attack with a .357 Smith & Wesson revolver.
The suspect faces charges of assault with intent to kill while armed and possession of a firearm during a crime of violence.
Pirro said that "it's too soon to say" what the suspect's motives were.
The charges could be upgraded, Pirro said, adding, "We are praying that they survive and that the highest charge will not have to be murder in the first degree. But make no mistake, if they do not, that will certainly be the charge".
The rare shooting of National Guard members on US soil, on the day before Thanksgiving, comes amid court fights and a broader public policy debate about the administration of President Donald Trump use of the military to combat what officials cast as an out-of-control crime problem.
The US administration quickly ordered 500 more National Guard members to Washington DC.
The suspect who was in custody also was shot and had wounds that were not believed to be life-threatening, according to a law enforcement official who was not authorised to discuss the matter publicly and spoke to AP on condition of anonymity.
A resident of the eastern Afghan province of Khost who identified himself as Lakanwal's cousin said Lakanwal was originally from the province and that he and his brother had worked in a special Afghan Army unit known as Zero Unit in the southern province of Kandahar.
He said he had last spoken to Lakanwal about six months ago.
The cousin said both brothers had moved to the United States in 2021.
He said Lakanwal had started out working as a security guard for the unit in 2012, and was later promoted to become a team leader and a GPS specialist.
Lakanwal, 29, entered the US in 2021 through Operation Allies Welcome, a program in president Joe Biden's administration that removed and resettled tens of thousands of Afghans after the US withdrawal from Afghanistan, officials said.
The initiative brought roughly 76,000 people to the US, many of whom had worked alongside US troops and diplomats as interpreters and translators.
It has since faced intense scrutiny from Trump's allies, congressional Republicans and some government watchdogs over allegations of gaps in the vetting process and the speed of admissions, even as advocates say it offered a lifeline to people at risk of Taliban reprisals.
Lakanmal has been living in Bellingham, Washington, about 127km north of Seattle, with his wife and five children, said his former landlord, Kristina Widman.
Prior to his 2021 arrival in the United States, the suspect worked with the US government, including the CIA, "as a member of a partner force in Kandahar," John Ratcliffe, the spy agency's director, said in a statement.
He said the relationship "ended shortly following the chaotic evacuation" of US service members from Afghanistan.
Trump on Wednesday night called for the reinvestigation of all Afghan refugees who entered under Biden's administration.
"If they can't love our country, we don't want them," he said, adding that the shooting was "a crime against our entire nation".
Washington DC mayor Muriel Bowser interpreted the shooting as a direct assault on the United States itself, rather than specifically on Trump's policies.
"Somebody drove across the country and came to Washington DC to attack America." Bower said.
"That person will be prosecuted to the full extent of the law."
