Attorney General Gordon J. MacDonald said a report released by St. Paul's School on past assaults, an investigation into recent reports of misconduct and the trial of Owen Labrie prompted the investigation.
"Protection of children is a paramount priority for law enforcement. I am confident that an institution such as St. Paul's School will be fully cooperative with this investigation," MacDonald said in a statement announcing the investigation.
Labrie, a former St. Paul's student was convicted in 2015 of a felony -- the use of an online service to seduce, solicit or entice a child under 16 in order to commit sexual assault -- and other charges stemming from an encounter at the school in Concord during the previous year. He was acquitted of more serious charges, including aggravated felonious sexual assault.
Claims of sexual misconduct at the school were the subject of a report commissioned by the school and released in May. Investigators found substantiated allegations of sexual assault by 13 former faculty or staff members at the school over a 40-year period ending in 1988.
Another investigation is ongoing after "students came forward (in June) and alerted SPS faculty to behaviors that were concerning to them," the school announced June 30.
"We have been in close contact with local law enforcement regarding recent incidents of concern, and we will continue to fully cooperate with any inquiries we receive," the school's Rector, Mike Hirschfeld, said in a statement Thursday. "We also intend to work closely with the attorney general's office to answer any and all questions regarding the independent report issued last month.
"Our goal is and always will be the health, safety and well-being of our students. We will work tirelessly to meet that goal and strengthen the public's faith in St. Paul's School."
The attorney general's office said it initially looked into whether the school endangered the lives of any of its students and whether the school obstructed criminal investigations.
"This office will investigate any other crimes as dictated by the evidence," MacDonald's office said.
Labrie's trial drew nationwide coverage
and brought attention to the now-161-year-old elite school, the alma mater of former Secretary of State John Kerry and half a dozen members of Congress.
A campus tradition known as the "senior salute" came under fire for allegedly encouraging seniors to have sexual encounters -- ranging from kissing to intercourse -- with as many younger female students as possible. The school said in 2015 that it never condoned sexual misconduct and that any game of sexual conquest was grounds for expulsion.
Labrie was an 18-year-old senior and his victim was 15. She testified that she didn't object to him kissing her but after he began to grope her she told him to stop. Then he raped her, she said.
Labrie, who recently lost an appeal for a new trial, said he stopped before having sex with the girl. He later bragged and lied about what happened to his friends, he testified.
The Episcopal school has more than 530 students and the cost of attendance is more than $60,000 per year. More than 40% of students receive financial aid.
The Merrimack County Attorney's Office, the Concord Police Department and the New Hampshire State Police are assisting in the attorney general's investigation.