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 following day. 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 14:00 EST (19:00 GMT) 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 guard 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 victims have been named as Sarah Beckstrom, 20, and Andrew Wolfe, 24 - both uniformed members of the West Virginia National Guard.
Ms Beckstrom died on Thursday, Trump said while on a Thanksgiving call with US service members.
He said Mr Wolfe remains in critical condition after being shot multiple times.
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.
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. He lived with his wife and five children on the other side of the country, in the state of Washington.
Mr Lakanwal reportedly came to the US in September 2021, 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. As the Taliban took back control of the country, there were fears of retribution against those who had co-operated with the US.
The Congressional Research Service has estimated that around 77,000 Afghans entered the US under special immigration protections, in place for about a year following the US withdrawal.
Mr Lakanwal had a relationship in Afghanistan with the US forces, FBI Director Kash Patel said. CIA Director John Ratcliffe said he had worked with the intelligence agency.
Mr Lakanwal applied for asylum in 2024, and his application was granted earlier this year, an official told CBS.
The US attorney for Washington DC, Jeanine Pirro, said the initial charges of assault against the suspect will be upgraded to murder in the first degree.
Initially, the administration had announced that Mr Lakanwal would be charged with three counts of assault with intent to kill while armed, along with possession of a firearm during a crime of violence.
"It is a premeditated murder. There was an ambush with a gun toward people who didn't know what was coming," Pirro told Fox and Friends on Friday. "And that is the homicide, and that is the murder that we're looking at right now."
Just hours after the shooting, Trump posted a video speaking from Mar-a-Lago in Palm Beach, Florida.
He described the shooting as an "act of terror" and "a crime against humanity".
Trump said he would deploy an additional 500 National Guardsmen to Washington DC - where there are currently nearly 2,200 troops.
National Guard troops are a reservist force that can be activated to serve as military troops, but have limited power as they cannot enforce the law or make arrests.
Trump deployed troops in August to tackle what he called "out of control" crime in the nation's capital.
Overall crime in the nation's capital has fallen since the force was sent, which Trump credits to the troops' presence on the streets.
The Trump administration has also highlighted the suspect's background, and announced a range of measures aimed at toughening the US approach to migration from Afghanistan and other nations.
The day of the shooting, it suspended processing all immigration requests from Afghans.
The next day, the US Citizenship and Immigration Services (USCIS) said it would re-examine green cards [permanent US residence cards] issued to individuals who migrated to the US from 19 countries.
And Trump went on to say he would "permanently pause migration" to the US from all "third-world countries" - using a term that was used in the past to describe poorer, developing nations.