St. Albans School and Washington National Cathedral in Northwest Washington. (Sarah L. Voisin/The Washington Post)

Washington National Cathedral and two District private schools said Thursday that they are joining a sexual misconduct investigation launched in February by an affiliated school that sought to document years-old allegations of inappropriate behavior.

In a letter to the school community last month, St. Albans School announced an investigation after hearing “firsthand accounts of inappropriate behavior and sexual misconduct by former St. Albans teachers.” The letter said there is no evidence of recent misconduct at the prestigious all-boys school.

St. Albans launched the investigation after The Washington Post reported allegations that teacher Vaughn Keith sexually abused a student at Key School, a private school in Annapolis, in the 1970s and was fired for alleged sexual misconduct. Keith was named in a January report commissioned by Key School that corroborated allegations against him, including those of a woman who said he sexually abused her while she was a student.

Keith went on to teach Latin and classical history at St. Albans for six years in the 1980s and died of complications from AIDS in 1990 at age 40. School officials have said they are not aware of inappropriate behavior involving Keith at St. Albans.

On Thursday, Washington National Cathedral, the all-girls ­National Cathedral School and the Beauvoir School announced they would join St. Albans’s investigation. The Episcopal institutions share a campus on Wisconsin Avenue in Northwest Washington.

“All four Close institutions are undertaking a comprehensive and independent investigation of any reports of past cases of sexual misconduct between adults and children at our institutions,” read a letter signed by the Rev. Randolph Marshall Hollerith, dean of Washington National Cathedral, and school officials.

In a separate letter, Hollerith said the cathedral “chose to join this investigation because it is the right thing to do.”

“Our responsibility is not only to protect children, but also to be what Isaiah called ‘the repairers of the breach,’ ” the letter said. “Through this investigation, we aim to offer healing to anyone who was hurt, and to try to repair any breach of trust that was broken.”

Washington National Cathedral and school officials declined additional comment.

In a letter Thursday to the Episcopal Diocese of Washington community, Bishop Mariann Edgar Budde said the decision to expand the investigation wasn’t prompted by reports of any current misconduct involving adults and students.

“Reckoning with our past and making amends to anyone who was harmed is not an easy path to take, and I give thanks for the leaders of Cathedral Close institutions who are showing us how to face a difficult issue with bravery, determination, and honesty,” she wrote.

The New York law firm Debevoise & Plimpton was hired to conduct the expanded investigation, which has included discussions with former students who reported abuse.

Stewart Patrick, 54, spoke weeks ago to an investigator with the firm for about an hour. He said he was abused by a St. Albans teacher as a rising eighth-grader in the summer of 1978.

Patrick, a former St. Albans student body president and Rhodes scholar who is a fellow at the Council on Foreign Relations, said the incident happened at the teacher’s apartment in suburban Maryland. Patrick’s brother, Bill, who works for New Brunswick’s Education Ministry in Canada, said the teacher gave him pornography as a seventh-grader and routinely made sexually inappropriate comments to students.

Debevoise & Plimpton declined to comment on the investigation, which includes three of Washington’s most lauded private schools and the faith community they call home.

The cathedral routinely hosts national prayer services and state funerals, including those of presidents Ronald Reagan and Gerald R. Ford and, more recently, senator John McCain. St. Albans, the National Cathedral School and Beauvoir are among the most exclusive schools in the Washington region.

The Rev. Mark H. Mullin, an Episcopal priest and headmaster of St. ­Albans from 1977 to 1997 who authored a book about the school, declined to comment last month on details of the St. Albans investigation.

“It’s a good thing for almost any school to do,” he said. “I think we’re all waiting until the report comes out.”

Mullin’s book, “The Headmaster’s Run,” published in 2008, said St. Albans’s status as an Episcopal school gave it “a special identity.” He described comforting Keith — the teacher accused of sexual abuse in Annapolis before his tenure at St. Albans — when Keith was diagnosed with AIDS in 1987.

“I got up, walked over to Vaughn, and hugged him,” he wrote. “As we embraced I realized that we had started down a path that would have a significant effect on St. Albans and well beyond.”

St. Albans has 575 male students in grades four through 12. Tuition for the current academic year ranges from about $45,000 to $65,000, and the school granted more than $4.5 million in scholarships during the 2017-2018 academic year.

The National Cathedral School has about 600 female students in grades four to 12, with tuition for the 2019-2020 academic year of about $45,000. Beauvoir is a coed elementary school with 390 students from prekindergarten to third grade and tuition that exceeds $36,000.

Julie Zauzmer contributed to this report.