Philadelphia police syaing... a suspected shoplifter was being questioned by a security guard in a Pathmark supermarket in Wynnefield Heights... when the man opened fire on the guard.

Shoppers and workers turned into witnesses as they watched the gunman shoot the guard three times-- once in the head and walk out the front store doors-- in front of the surveillance cams. Police are using that vid to try to figure out who he is and find him.