They get information that he/she is in a crowded market. So they quickly use facial recognition system over the market’s CCTV footage to identify the person. In reality, facial recognition doesn’t work that fast. It works based on some algorithms and could only be used to match separate pictures like passport, or ID card.
Secondly, identifying a dead person’s face through a photograph is not that easy as his/her eyes are usually shut. Many biometric systems won’t start monitoring without eyes. And for several other reasons like light conditions, moving angles, distances, hairstyles, etc. finding a match is tough.