Alicia Upton paced the concrete floor of her jail cell. She looked around the cramped quarters. Then she pressed the alert button on an intercom attached to the wall.

“What is your emergency?” responded a voice, captured on video footage from a camera in the cell. It was a deputy about 50 feet away, in the control room of the women’s mental health unit where Ms. Upton, 21, was being held.

“It’s not an emergency, but —” she began, then the deputy cut off the call before she could finish. Charged with a misdemeanor, Ms. Upton was awaiting a court-ordered evaluation to determine whether she was competent to stand trial.

She took a few more listless steps, the video shows. She paused beneath a buzzing fluorescent light, then picked up a white bedsheet and said, “It’s time to hang myself.”

She was found, limp, 20 minutes later. In the interim, the camera recorded the young woman preparing to end her life. But no guards, who were tasked with monitoring the video feed, noticed until it was too late.