(CNN) A mural celebrating African-American achievement was defaced with swastikas on Thursday in Los Angeles, police said. They are investigating the incident as a possible hate crime.

Four swastikas were spray painted over the faces of four Black Panthers featured in the mural in southwest Los Angeles, said LAPD officer Jeff Lee.

Investigators believe it is an isolated incident, Lee said. No arrests have been made.

A dozen artists had painted the two-block long mural, entitled "Our Mighty Contribution," over two years, wrapping up in 2002, said Gregory Everett, a filmmaker and the curator of the mural. Located on the busy Crenshaw Boulevard, the mural chronicles the historical contributions of black people, Everett said.

Over the past 18 years, only a few people have scribbled their names on the wall, Everett said.

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