Two further victims of the incident were hospitalized for non-life-threatening wounds.
According to authorities, the assault began on a city bus and concluded on the platform of a Metro station in Southeast Washington.
Bystanders disarmed the suspect, which led to his capture.
The Washington Metropolitan Area Transit Authority expressed its sorrow over the passing of "heroic employee, Robert Cunningham, who intervened on behalf of a client" at Potomac Avenue Station in a statement on Wednesday.
It stated that Mr. Cunningham, 64, was "a victim of senseless gun violence" and worked as a mechanic for the transportation authority's power division.
Isaiah Trotman, 31, of Southeast Washington, the suspect, was detained and charged with first-degree murder while carrying a weapon, kidnapping while carrying a weapon, and assault with a dangerous weapon, according to the police.
On Wednesday morning at around 9:00 EST (14:00 GMT), the shooting took place, according to the police.
According to assistant police chief Ashan Benedict, the man was "brandishing" a firearm and "engaging" other commuters while riding a Metro bus.
Following another passenger off the bus, the shooter shot him in the leg before going into the Potomac Avenue Metro Station and shooting a second victim in the leg, according to authorities.
He attempted to "rob, confront, or otherwise have an argument with" a woman as he descended the platform by aiming his revolver at her.
Mr. Cunningham, a Metro employee, then made an attempt to "intervene to protect this young lady" before being shot and killed there and then.
The gunman was eventually disarmed by commuters and a second employee, who both assisted in the intervention. The gunman was then apprehended by responding police officers.
The idea that our folks had to confront armed gunmen is troubling to me, Mr. Benedict remarked, but I believe they (commuters) saved lives, and that's something to be appreciated.
When the shooting started, passengers who were on the Metro described the scenario as "hysterical."
A conductor on a Metro train instructed everyone to get down on the ground and turn out the lights during the shooting, according to one eyewitness who spoke to the Washington Post. Another person claimed the shooter shouted "I'm the killmonger today" and "you be safe today" as he strolled past commuters.
Minutes after the incident, Michael Campbell, who was on the platform, told the local news station WUSA9 that "people were shouting, people were running."
"It's unheard of and strange that someone would run down onto the platform and shoot three people. Life is not respected, "added Mr. Campbell.
Police are currently looking into the shooting and the shooter's intentions.
The Washington, DC, transportation authority announced that it has lowered its flags to half-staff in honor of Mr. Cunningham.
BBC
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