Police Commissioner Raymond W. Kelly said detectives are working under the premise there is a single gunman since all three victims were of Middle Eastern origin, the New York Times reported Tuesday.
In addition, Kelly said "the possibility of a bias motive" is something that can't be excluded.
Similarities in all three homicides led police to suspect the same killer. However, Brooklyn District Attorney Charles J. Hynes said there does not appear to be anything tying the three victims to each other.
"There is nothing about the victims that would suggest anything other than they were just the victims of a random execution," Hynes said.
Surveillance video from the area where the latest victim, Rahmatollah Vahidipour, was shot to death in his Flatbush Avenue shop produced images of four people authorities wanted to question.
By Monday, police had narrowed their focus to a mustached man carrying a duffel bag, WPIX-TV, New York, reported.
Profilers from the Federal Bureau of Investigation are helping New York City detectives, Commissioner Kelley said.