Two members of the National Guard were shot and seriously wounded on Wednesday just blocks from the White House in Washington DC.
One died on Thursday while the second remains in a critical condition, US President Donald Trump said. The suspect, an Afghan national, is in custody.
In response to the attack, Trump said he would deploy another 500 troops to the streets of the nation's capital.
Just after 14:00EST (19:00GMT) on Wednesday, two members of the National Guard, both from West Virginia, were shot at close range near Farragut Square in downtown Washington DC.
The soldiers had been on a high-visibility patrol near the corner of 17th and I streets, an area where many office workers mill around at lunchtime.
Metropolitan Police Assistant Chief Jeff Carroll said the suspect 'came around the corner' and 'immediately started firing', adding that the soldiers had been 'ambushed'.
Muriel Bowser, the mayor of Washington DC, said the incident was a targeted attack.
The location of the shooting, just blocks from the White House, meant a number of law enforcement officials were quickly on the scene to treat the two victims and apprehend the gunman.
The guards members were taken to hospital, where one later died.
Trump was at his Mar-a-Lago residence in Florida ahead of the US Thanksgiving holiday and not at the White House at the time.
The Department of Homeland Security named the suspect as Rahmanullah Lakanwal, and described him as 'a criminal alien from Afghanistan'.
During his apprehension, the suspect was shot four times, law enforcement sources told CBS News, the BBC's US news partner.
Mr Lakanwal is 29 years old and not co-operating with authorities, officials said.
He is said to have come to the US in 2021, reportedly under a programme for Afghans named Operation Allies Welcome that existed during the presidency of Joe Biden.
This was during the time period that followed a chaotic withdrawal of US forces from Afghanistan.
The Congressional Research Service estimated that around 77,000 Afghans entered the US under special immigration protections, in place for about a year following that withdrawal.
Mr Lakanwal had a relationship in Afghanistan with the US forces, FBI Director Kash Patel said on Thursday. CIA Director John Ratcliffe said he worked with the intelligence agency.
Mr Lakanwal applied for asylum in 2024, and his application had been granted earlier this year, an official told CBS.
In the wake of the suspect's connection to Afghanistan, Trump has said that the US 'must now re-examine every single alien who has entered our country from Afghanistan under Biden'.
Mr Lakanwal will be charged with three counts of assault with intent to kill while armed, US Attorney Jeanine Pirro said. He will also be charged with possession of a firearm during a crime of violence.
The first charges mean he will face 15 years behind bars if found guilty.
The victims have been named as Sarah Beckstrom, 20, and Andrew Wolfe, 24.
They are both uniformed members of the West Virginia National Guard, Pirro said.
Attorney General Pam Bondi told Fox News that Ms Beckstrom had volunteered to work in the nation's capital over the Thanksgiving holiday. 'She volunteered, as did many of those guardsmen and women, so other people could be home with their families,' she said.
Ms Beckstrom died on Thursday, President Trump said during a Thanksgiving call with US service members.
Mr Wolfe remains in critical condition after being shot multiple times.
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