Baltimore Swastikas and racial epithets were spray-painted onto a mural of abolitionist Harriet Tubman.
The Harford County Sheriff's Department had no suspects, and officers were canvassing the area, Lt. Ed Hopkins said.
The 18-foot-by-50-foot painted mural, on the side of the Magnolia Middle School, is one of five works of Tubman commissioned from artist Mike Alewitz for public buildings around the state.
Tubman, known as "the Moses of her people," was born a slave in 1820 and escaped to the North in 1849 on the Underground Railroad, the secret network of safe houses set up to help slaves flee. Over the next decade she led nearly 300 Maryland slaves to safety.
The mural in Magnolia, a Baltimore suburb, depicts Tubman on a computer screen, her hands outstretched across a book. There is a swirling red sea, evoking the biblical story of Moses leading the Israelites out of slavery by parting the Red Sea.
A custodian discovered the damage Thursday morning, school spokesman Don Morrison said. The mural will be repaired.
"Sometimes, it's hard to tell whether someone is acting out of maliciousness or out of hate," Hopkins said. "But given the nature of the defacing, we clearly feel that the remarks were inflammatory, and therefore, are investigating it as a hate crime."
Twenty-five percent of the middle school's students are black. The surrounding neighborhood is closer to 50 percent black, Morrison said.