A campus e-mail from Harvard Law School’s dean for administration said police conducted numerous interviews and performed a forensic examination. But they could not identify who placed diagonal slashes of black tape on the glass of several photo portraits in a law school building, or why they did it.

The e-mail added that it remains a “deeply troubling incident”.

Only portraits of black professors were targeted in the vandalism, which was reported on November 19 at Wasserstein Hall and heightened racial tensions at the Ivy League university.

The e-mail said Harvard plans to install security cameras in the building in response to the vandalism.