A man has been arrested for fatally stabbing a woman and wounding another on a California train, in what appeared to be a random attack, officials say.
Nia Wilson, 18, was killed after she and her sister were both stabbed in the neck while changing trains on Sunday night in Oakland.
Police say John Lee Cowell, 27, struck the women in a way "more reminiscent of a prison yard assault", then fled.
Wilson's death is the third linked to the Bay Area rail in less than a week.
In a statement, the Bay Area Rapid Transit system (Bart) said Cowell was arrested on Monday night onboard another train after an anonymous tipster called police.
Bart police chief Carlos Rojas said in a news conference that CCTV footage shows that the women did not interact with Cowell as they rode the train together to the city's MacArthur Station.
"It basically happened at the snap of the fingers, at the drop of the pin," he said, adding that the attack was "the most vicious" he had seen in his nearly 30-year police career.
"It's more reminiscent of a prison yard assault," Chief Rojas said. "They do their attack so quickly that before anybody can really react, the person takes off running."
After the attack, surveillance cameras captured the attacker shedding his clothes in a nearby Bart car park. The knife was later discovered at a construction site.
Police have yet to establish a motive for the attack.
Officials said it was unclear if race played a role in the stabbing. Cowell is white and the women he is accused of stabbing are black.
Cowell - who police described as having a "violent past" - has previously been convicted of second-degree robbery, battery, being under the influence of a controlled substance, vandalism and petty theft.
In a statement, Oakland Mayor Libby Schaaf said that although the attacker's motivation is not yet known, "the fact that his victims were both young African-American women stirs deep pain and palpable fear in all of us who acknowledge the reality that our country still suffers from a tragic and deeply racist history".
Speaking outside a relative's home, Letifah Wilson, 26, said that she and her sister were returning home from a family event when they were "blindsided by a maniac".
"He didn't know us, we didn't know him," said Ms Wilson, who was injured in the attack. "For what? I don't know why.
"And I looked back, and he was wiping off his knife and stood at the stairs and just looked - and from there on, I was just caring for my sister.
"I was in shock... I didn't know I was cut because I was paying more attention to my sister. But he just stood there, like it was nothing."
Alicia Greyson, the victim's mother, described her as a "wonderful" daughter who had been a cheerleader and basketball player at the Oakland high school.
"This man just came up and did all this to my baby," she said through tears.
She added that Wilson had a job interview scheduled for Monday.
"She was so smart and gifted," her mum added.
On Monday night, hundreds of people gathered for a vigil at the station where the women were attacked, and later marched downtown.
The fatal stabbing was the third Bart-related death in the last five days.
On Sunday, a homeless man died after he was punched and fell down, hitting his head on the pavement.
On Wednesday, a 51-year-old Pittsburgh man sustained a bloody lip and small cut on his leg after getting into an argument with a 20-year-old passenger.
On Friday, he was discovered dead back home in Pittsburgh. A post-mortem examination concluded that he died from an infection stemming from the cut on his leg.