MIAMI BEACH, Fla. — In the bustling South Florida high school where she was a senior, Sol Pais blended into the background, a quiet presence who took honors English and Advanced Placement studio art classes and did not strike classmates or teachers as deeply troubled.

Instead, Ms. Pais, 18, seemed to share her darkest thoughts online.

Images of a handwritten journal, signed with Ms. Pais’s full name, showed a young woman in prolonged despair. Journal entries dated over the past year included admissions to harboring increasingly “extreme” views and to looking forward to acquiring a firearm.

She felt, Ms. Pais appears to have written, like a pot of scalding water “on the verge of boiling over.”

On Wednesday, the police found Ms. Pais dead of an apparent self-inflicted shotgun wound near the base of Mount Evans in Jefferson County, Colo., more than 2,000 miles away from home. Officials in Colorado had been searching for her for 24 hours, saying they believed her to be “infatuated” with the mass shooting at Columbine High School in April 1999, and that she had flown to Denver on a “pilgrimage.” Hundreds of Colorado schools were closed on Wednesday as a precaution.