(CNN) -- Nine Massachusetts teenagers have been charged with involvement in a months-long campaign of bullying that led to the suicide in January of a 15-year-old girl, a prosecutor said Monday.

Phoebe Prince's body was found hanging in the stairway leading to her family's second-floor apartment in South Hadley, Northwestern District Attorney Elizabeth D. Scheibel told reporters in the western Massachusetts town of Northampton.

"It appears that Phoebe's death on January 14 followed a torturous day for her when she was subjected to verbal harassment and physical abuse," she said.

Earlier in the day, Prince had been harassed as she studied in the library at South Hadley High School, apparently in the presence of a faculty member and several students, none of whom reported it until after the death, Scheibel said.