Three staff members at an elite prep school near Houston, Texas, sexually abused 15 students over three decades, according to the results of a 15-month investigation released on Thursday by the school’s board of trustees.

The investigation at The Kinkaid School, which currently has over 1,000 students from pre-kindergarten through 12th grade, involved interviews with at least 58 people by private security firm T&M Protection Resources. The school announced in May 2018 that it had hired the company to probe allegations of sexual misconduct between the late 1960s and early 1990s. The board of trustees’ summary of the investigation’s findings was published in full by ABC 13.

“We are deeply disappointed by the school’s failure at times in its past to provide a safe and secure environment, and we offer our most sincere apologies to those students who suffered as a result,” said the board of trustees in a statement on Thursday.

The company concluded that one faculty member committed sexual misconduct, including inappropriate touching, with eight students between the 1960s and early 1970s, according to the investigation. Those students were all between the ages of 10 and 13.

In a second case, another faculty member engaged in inappropriate touching, attempted to sexually assault a student, and solicited students to engage in sex acts between the late 1980s and early 1990s, according to the report. There were six student victims of that faculty member, ranging between the ages of 14 and 17.

A third faculty member engaged in “an inappropriate relationship,” which involved physical contact and sexual interaction with a 16-year-old student in the 1990s, according to T&M’s findings.

T&M also found that the prep school’s senior leadership was made aware of one of the instances of misconduct toward a student and failed to terminate that faculty member, instead implementing “remedial steps.” The other two staff members were immediately terminated following the allegations.

The investigation included interviews with alumni, current and former faculty, staff, administrators, and others, and Kinkaid said in its report that the school “provided the investigators with unrestricted access to school-affiliated individuals, files and other school resources.”

“We affirm our deep gratitude to the survivors and witnesses who came forward, and we pledge our continued commitment to students, parents, faculty and staff, and alumni to promote educational excellence, personal responsibility and a safe and respectful learning environment,” the board of trustees said in its statement.

Kinkaid reported the findings of the investigation to local authorities but “made the decision not to disclose the names of the three former faculty members,” according to the board. Survivors were purportedly notified in advance of Kinkaid’s plan to release the findings.

“This behavior is simply not tolerated at Kinkaid today,” the school said, noting that T&M did not uncover any allegations involving current faculty or staff members.

“We as a school community can and will do better to protect our students.”