unknown persons?

""... unknown persons detonated a bomb under the monument, which is located at Abu Ali Mustafa Square in the Be'r al-Na'ja area of Jabalya city. The monument depicts Abu Ali Mustafa as well as George Habash, founder of the PFLP. The explosion resulted in the destruction of
Habash's portrait. Ten days earlier, unknown persons had painted the monument with tar"" (thanks Asa)