LARAMIE, WYO. — After her family moved from suburban New Hampshire to the wind-whipped plains of southeastern Wyoming, Monica Morin embraced small-town life, forging lasting friendships and celebrating her own quirky style.

Dark-haired, with hipster glasses and a disarming sense of humor, Monica was a “why-not kind of kid,” her mother, Kim Morin, said. The kind who would wear a giraffe costume to the grocery store, just because.

Last year, during Monica’s sophomore year of high school, her mood began to darken. She turned to alcohol and marijuana, and some days withdrew from the close relationship she had always had with her parents, who, although long divorced, remained friends and partners in raising their only child. After her descent into drinking, she started cutting herself.

Ms. Morin was alarmed, aware that family history was not in her daughter’s favor. Her sister had developed bipolar disorder in her teens, eventually drinking herself to death. Her father had taken his own life when Ms. Morin was 19.