(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.

Prince, who had recently moved to the area with her family from Ireland, was also harassed as she walked through the halls of the school that day and as she walked on the street toward her home, Scheibel said.

The harassment that day, by one male and two females, "appears to have been motivated by the group's displeasure with Phoebe's brief dating relationship with a male student that had ended six weeks earlier," she said.

But that day's events were not isolated; they "were the culmination of a nearly three-month campaign of verbally abusive, assaultive behavior and threats of physical harm toward Phoebe on school grounds by several South Hadley students," Scheibel added.

"Their conduct far exceeded the limits of normal teenage relationship-related quarrels. The investigation revealed relentless activity directed toward Phoebe designed to humiliate her and to make it impossible for her to remain at school."

In the indictments, returned Friday but not made public until Monday, the Hampshire County grand jury charged 17-year-old Sean Mulveyhill of South Hadley with statutory rape, violation of civil rights with bodily injury resulting, criminal harassment and disturbance of a school assembly.

The indictments charged 18-year-old Austin Renaud of Springfield with statutory rape.

Kayla Narey, 17, of South Hadley, was charged with violation of civil rights with bodily injury resulting, criminal harassment and disturbance of a school assembly.

CNN was not able to reach any of the three, none of whom has been taken into custody, according to Scheibel. She did not elaborate on the statutory rape charges.

Charges against another three girls included violation of civil rights with bodily injury resulting; two were also charged with stalking.

Three other girls from South Hadley were named in four delinquency complaints from Hampshire Franklin Juvenile Court. Their charges included violation of civil rights with bodily injury resulting, criminal harassment and disturbance of a school assembly, violation of civil rights, criminal harassment and assault by means of a dangerous weapon. One of the juveniles was charged in a separate complaint involving a second victim, Scheibel said.

Though initial news reports blamed Prince's suicide on cyberbullying, Scheibel said the students' actions were "primarily conducted on school grounds during school hours and while school was in session." She said any use of electronic social networks was secondary to "commonly understood bullying methods."

The bullying of Prince was common knowledge to most of the student body and to certain faculty, staff and administrators, Scheibel said. At least four students and two faculty members had intervened during the harassment, but the school's code of conduct was inconsistently enforced, she said.

Though the faculty, staff and administrators' behavior was not deemed criminal, "the actions, or inactions, of some adults at the school are troublesome," she said.

Scheibel said the Prince family had asked "that the public refrain from vigilantism in favor of allowing the judicial system an opportunity to provide a measure of justice for Phoebe." CNN was not able to reach the Prince family.

Scheibel said the investigation was continuing and that a 10th person may be charged.

Arraignments will occur "in the near future," she said.

Several of the students remain in the school. In a statement, South Hadley Public Schools Assistant Superintendent Christine Sweklo said school officials would meet with the district attorney to review any new evidence.

"Once we are able to obtain this information, we will be able to make a more comprehensive statement and possibly take further action against the students still attending South Hadley High School," she said.

The statutory rape charges could result in penalties of up to life in prison, said Elizabeth Farris, a lawyer in the prosecutor's office. A conviction on a violation-of-civil-rights charge could result in 10 years in prison; a conviction for criminal harassment up to 2 1/2 years; a conviction for disturbance of school assembly up to one month, she said.

Those 17 and older will be tried as adults in Hampshire County Superior Court. The juveniles will be tried in juvenile court. A juvenile court judge may choose to institute an adult penalty, a combination of adult and juvenile penalties or a juvenile penalty alone, Farris said.

