Her 18-year-old sister Nia Wilson passed away at the scene. BART police are now looking for Cowell.
"I'm her protector and I feel like I didn't protect her," said Wilson.
"She's just yelling my name, 'Tifah, Tifah, Tifah,' and I said, 'I got you baby, I got you,'" said Wilson as she described the unprovoked stabbing.
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Wilson said she responded by telling her sister she loved her.
"We're gonna get through this, I got you, you're my baby sister," said Wilson.
In describing her sister Nia, she said, "The most sweetest person on the earth. She didn't do nothing to nobody. I didn't do nothing to nobody and for this to happen is just outright crazy."
Wilson says she was with two of her sisters, including Nia, heading home from a family function. She says they never saw Cowell or exchanged words with him prior to the stabbing.
"All of a sudden we transfer, just to get blindsided by a maniac, for what I don't know," said Wilson.
"I looked back and he was wiping off his knife and stood at the stairs and just looked. From then on I was caring for my sister," continued Wilson.
Wilson says a woman with a stroller that she and her sister had allowed to pass earlier gave her a baby blanket to apply pressure to her sister's neck.
Wilson says she was also hurting because Cowell had stabbed her after stabbing Nia.
She says when other BART passengers attempted to stop Cowell, he ran from the scene.
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Describing her sister, Wilson said, "That's my heart, she was my heart. No matter our ups and downs we still come back to each other like it was nothing."
She says the family hadn't seen Nia in six months because she was with her mom's side of the family. Wilson also says they rarely took BART.
"As young black women, we shouldn't have to look behind our back, you know. We should be living freely like everybody else," said Wilson.
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"He just felt upon himself to take his anger, or whatever the case may be, out on innocent women," Wilson continued.
Nia's nickname, according to her family, was PG, short for Pretty Girl. They say she loved to spend lots of time getting ready and doing her makeup.
Wilson says her baby sister wanted to be a lawyer or work in criminal justice. But she had more aspirations, like having a dance studio and her own makeup line.
As for her own injuries, Wilson has stitches in her neck.
"Missed the artery, thank God, missed the veins. It's pretty deep and he tried to swing twice hit me in my head, so I have a knot in my head," said Wilson.
Still she's focused on Nia.
"I want justice for my sister," said Wilson.
Murder suspect identified in MacArthur stabbing. John Cowell is considered dangerous. Please call 911 if spotted. RIP Nia Wilson. We will bring this suspect to justice. Our thoughts are with the family and friends. Details: https://t.co/DEkTJvJnNe pic.twitter.com/igZU2bdel7— SFBART (@SFBART) July 23, 2018
Ahead of tonight's vigil for BART stabbing victim Nia Wilson pic.twitter.com/N2klP0BlZQ— Melanie Woodrow (@MelanieWoodrow) July 23, 2018
Here are the 5 siblings. They tell me they hadn't seen Nia Wilson in 6 months. The family had just come back together for a family gathering. pic.twitter.com/NBJii1ramv— Melanie Woodrow (@MelanieWoodrow) July 23, 2018