Detective John O'Connor says the suspect put his revolver to the head of the victim, Christopher Small, of Brooklyn, N.Y., and shot him at least three times.
Suspect Leonardo Olmeda-Pagan is still at large.
The 33-year-old Olmeda-Pagan and several other men showed up uninvited at a birthday party late Saturday.
When an iPod was discovered missing from the party, Small helped the birthday girl's father to clear out the party.
Witnesses say that angered Olmeda-Pagan.
O'Connor says Olmeda-Pagan followed Small to his car afterward and shot him on the sidewalk.
