It was a November afternoon in 2018 that Grace McKinnon first met Anthony Williams. The licensed social worker was on her way to her office, according to multiple news reports, when she saw a young black man engaged in a confrontation with U.S. Capitol Police near Union Station. She immediately stopped her car and got out to record, later posting video of the encounter on Twitter.

“I knew I had to help. I saw it happen,” McKinnon told The Root in 2018. “I had to get involved.”

McKinnon recorded as she helped calm an obviously distressed Williams, who was at first ignoring orders from an officer shouting at him to get on the ground, gun drawn. “Just get on the ground, bruh. Hey bruh, just get on the ground. Don’t die, this is not worth dying for,” she told a tearful Williams. He listened to her, rolling over onto his stomach. McKinnon then helped guide his hands behind his back so that officers could handcuff him.

“I literally think I just saved someone’s life… I’m shaking y’all…” McKinnon later posted on Twitter, along with the video. The footage was retweeted tens of thousands of times. After the arrest, McKinnon befriended the then-19 year old, who’d been homeless since losing his mother to colon cancer two years prior. She raised more than $25,000 on GoFundMe to help him get back on his feet.

Hardly more than a year later, Williams was killed in a stabbing at Union Station on Sunday. The dispute that cost him his life was reportedly over a $5 debt.

“We were ready to turn his life around,” McKinnon told WUSA9 in an interview about Williams’ death on Monday. “But now I’m numb. I can’t process it. He’s dead now.”

Williams was just 20 years old. At the time he met McKinnon, he was still a high school student at Kingman Academy Public Charter School. In a March 2018 video by the organization Invisible People, Williams said he first became homeless after his mother’s death, and struggled to get a job and put a roof over his head. “Hopefully once I get myself together I want to build an orphanage or something like that,” he said in the video. “A free open house, for kids, adults, whoever. People that don’t have nowhere to go or people that’s been neglected. I want to give back.”

Williams’ family reached out to McKinnon and told her about his death on Monday, per WUSA9. The 20 year old was stabbed on the Metro platform at Union Station on Sunday. Witness accounts and surveillance video show a woman attacking Williams with a knife and stabbing him in the left shoulder, court documents say. Williams ran up the escalator and collapsed on the mezzanine level, where he was later pronounced dead.

Twenty-three-year-old Angel Moses was arrested in Dupont Circle later the same day in connection with the stabbing. Surveillance video reportedly shows her handing the knife off to another person, and then entering both the men’s and women’s restrooms at Union Station, where blood evidence was discovered. A man named Edward Everstine was also arrested in connection with the stabbing, for accessory after the fact—police reportedly found a bloodied kitchen knife in his duffel bag, per court documents.

Moses told police that she had wanted to buy K2, or synthetic marijuana, from Williams, but that he had refused to sell it to her because she owed him $5 from a previous transaction, per court documents. She also said she was only trying to threaten Williams, and that she didn’t know she’d stabbed him, the documents say. She has been charged with second degree murder and her preliminary court date is on February 6.

“He was so young. He still had his whole life ahead of him,” McKinnon told WUSA9 about Williams. “For it to end like this is like … I don’t know if there’s meaning in that. I don’t know if there’s meaning in that.”