Before she was arrested and accused of illegally obtaining the personal data of over 100 million people from Capital One, Paige Thompson, 33, had a public Twitter persona typical of a software engineer in Seattle.

She commented often on programming chatter, fretted about her dating life and mourned the euthanasia of her cat, Millie. Millie’s death, she wrote, was “one of the most painful and emotionally overwhelming experiences I’ve had in my life.”

But Ms. Thompson also spoke darkly about her mental health, writing on July 5 that she intended to check herself into a facility for treatment.

“I have a whole list of things that will ensure my involuntary confinement from the world,” she wrote. “The kind that they can’t ignore or brush off onto the crisis clinic. I’m never coming back.”