His brother saw him do the deed and has never recanted.
The Far Left has made Mumia an icon and elevated him to political prisoner status. Hence, at their protests, one can see "Free Mumia" t-shirts and signs and the like.
Hence "Fry Mumia." It drives 'em absolutely ballistic. The lefties froth at the mouth at the mere mention of this. I have videotape to prove it.