Philadelphia's Mural Arts Program is trying to figure out how one of its distinctive murals disappeared into black.

A tribute to the midcentury, African-American artist Dox Thrash that was painted on the side of a building at 24th Street and Cecil B. Moore Avenue has been completed covered with black paint.

The mural had been part of the Mural Arts Program's African-American collection, a curated tour of thematically linked murals. It is included in an audio tour, on which artist Richard Watson remembers learning about Thrash as a student at the Pennsylvania Academy of the Fine Arts.

"The instructor, in his own coarse way, said, 'He was one of your people,'" Watson says on the recording. "I assumed he meant he was a relative, but really he meant he was African-American. We were all proud to know that."

Now, all that's left of the mural is that audio segment. Just after Thanksgiving, the Mural Arts Program discovered the entire mural had been blacked out. They are scrambling to find out why.

Since September, the building has been owned by the U.S. Housing and Urban Development agency. A HUD spokeswoman, Linda Wolf, said the damage occurred before the agency acquired the building, as a foreclosure property, from City Mortgage

Wolf said City Mortgage likely painted over the mural because it had been defaced with graffiti after it acquired the property in May, as a mortgage default.

The Mural Arts Program is investigating those claims, and why it took six months to be discovered. Jennifer McCreary of Mural Arts said the program cannot monitor all of their murals without feedback from neighbors.

"We've created about 3,600 murals throughout the city," said McCreary. "Without someone reporting back to us about damage to a mural or graffiti on a mural -- that's what we rely on -- getting tips back when mural damage happens and we can get out there quickly to address it, clean it, repair it."

The Mural Arts Program wants to re-create the Dox Thrash mural, but because the building is now owned by HUD, the future of the wall is not considered secure. McCreary says the Thrash image is important to the Mural Arts collection and the organization is considering creating it on a different wall.