Portraits of several black professors were found vandalized at Harvard Law School on Thursday, and the Harvard University Police Department is investigating the incident as a hate crime, according to the dean who called racism at the school "a serious problem."

Black strips of tape were placed diagonally over at least five photo portraits, Harvard officials said. Students and professors noticed the tape as classes met on Thursday morning at Wasserstein Hall, which houses two hallways lined with framed portraits of more than 180 professors.

All of the portraits that were targeted depict black professors, but some portraits of black professors appeared to be untouched. Harvard officials said they don't know why some professors were singled out or who was responsible.

"Expressions of hatred are abhorrent, whether they be directed at race, sex, sexual preference, gender identity, religion or any other targets of bigotry," the law school's dean, Martha Minow, said in a statement.

The statement added that the vandalism was being treated as a hate crime.

Harvard police told the campus newspaper, the Harvard Crimson, that an investigation into the incident is “active and ongoing.”

The discovery stirred the campus a day after students rallied in support of black students across the United States. Racially charged protests at the University of Missouri this month touched off a wave of demonstrations and frank discussions about race at many other campuses.

Harvard students have called on the law school to open a diversity office, among other demands. And some students are pushing the law school to change its official seal, which is borrowed from the family crest of an 18th-century slaveholder whose estate helped found the school.

A university spokeswoman said she didn't know if there's a connection between the recent protests and the vandalism.

Some students said they were disturbed by the vandalism, calling it an overt display of racism.

"I know what it looks like when you're being told to get back into your place, and I think that is very much what this looks like," said Rena Karefa-Johnson, a third-year law student and a leader of Students For Inclusion, an activist group supporting minority students at the law school.

Leland Shelton, a third-year law student, said the vandalism is one more thing to make black students feel unwelcome.

"This kind of proves that we're not just making this stuff up," said Shelton, who is president of the Harvard Black Law Students Association.

Amid pressure from students, the law school's dean called an open meeting on Thursday to discuss the vandalism and issues tied to race.

At the meeting, Minow said racism is a “serious problem” at the school, according to the Crimson.

By Thursday afternoon, the tape had been removed from the glass over the portraits, and students were sticking notes of support around the photos.

Al Jazeera with The Associated Press