Duke Ellington School of the Arts informed parents Wednesday of an investigation into an alleged assault that happened in 2013. (Cheryl Diaz Meyer for The Washington Post)

D.C. police are investigating allegations of sexual misconduct against a department chair at Duke Ellington School of the Arts.

Principal Sandi Logan informed parents Wednesday in a letter that the teacher was placed on leave while the school investigated an alleged incident from 2013.

The investigation was prompted by a letter posted on social media this week from a former student accusing the teacher of sexually assaulting an underage student.

“You may be aware that an email has recently been circulated alleging sexual misconduct by a Duke Ellington staff member in 2013,” Logan wrote in her letter to parents. “I want to assure you that the safety of our students is our number one priority.”

D.C. police said the investigation is open.

While Duke Ellington School of the Arts is part of D.C. Public Schools, it has an unusual governance structure and its teachers are not represented by the union or considered city employees.

Since 2000, Ellington has been run jointly by D.C. Public Schools, the nonprofit Ellington Fund, the John F. Kennedy Center for the Performing Arts and George Washington University. It is funded by taxpayers but overseen by an independent board of directors.

Logan referred questions about the investigation to D.C. Public Schools.

Shayne Wells, a spokesman for the school system, said it is assisting with the investigation.