A transport worker was killed after trying to stop a gunman from shooting commuters in Washington DC on Wednesday morning, authorities said.
Two other people were wounded in the shooting and were treated in hospital for non-life-threatening injuries.
The attack started on a city bus and ended on the platform of a Metro station in Southeast Washington, police said.
The suspect was disarmed by bystanders and arrested.
In a statement on Wednesday, the Washington Metropolitan Area Transit Authority said it mourned the loss of a "heroic employee, Robert Cunningham, who intervened on behalf of a customer" at Potomac Avenue Station.
It said Mr Cunningham, 64, worked as a mechanic in the transit authority's power department and was "a victim of senseless gun violence".
Police said the suspect, Isaiah Trotman, 31, of Southeast Washington was arrested and charged with first-degree murder while armed, kidnapping while armed, and assault with a dangerous weapon.
The shooting unfolded at around 09:00 EST (14:00 GMT) on Wednesday morning, police said.
The suspect had been riding a Metro bus while "brandishing" a weapon and "engaging" other commuters, assistant police chief Ashan Benedict said.
The gunman followed a fellow passenger off the bus and shot him in the leg, and then entered the Potomac Avenue Metro Station where he shot another person in the leg, police said.
As he made his way down the platform, he pointed his gun at a woman and tried to "rob, confront or otherwise have an altercation with her".
Metro employee Mr Cunningham then attempted to "intervene to protect this young lady" before he was shot and killed at the scene.
A second employee also helped intervene, as did commuters who were able to disarm the gunman, who was then detained by responding officers.
"I think they (commuters) saved lives and that's to be commended, but the fact that our citizens have to intervene with armed gunmen is disturbing to me," Mr Benedict said.
Passengers who were on the Metro when the shooting unfolded have described the scene as "hysterical".
One eyewitness told the Washington Post that during the shooting, a conductor turned off the lights on a Metro train and asked everyone to get down on the ground. Another said the gunman uttered "you be safe today" as he walked past commuters, and said "I'm the killmonger today".
Michael Campbell, who was on the platform minutes after the shooting, told local network WUSA9 that "people were screaming, people were running".
"Somebody run down on the platform and shoot three people; it's unheard of - it's crazy. There's no regard for life," Mr Campbell said.
Police are now investigating the incident and the shooter's motive.
The transport authority in Washington DC said it has lowered its flags to half-staff in remembrance of Mr Cunningham.
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