The victims include famed forensic psychiatrist Steven Pitt, who was involved in several high-profile cases, including the investigation into the death of child beauty queen JonBenét Ramsey in 1996. He was found shot dead in Phoenix on Thursday after witnesses heard a loud argument and gunshots, officials said.
The other victims were 48-year-old Veleria Sharp and 49-year-old Laura Anderson, two paralegals at a Scottsdale law firm who were killed Friday afternoon.
They also were shot, police said in a news release.
"We are in the middle of our investigations so I'm not going to talk about who's being investigated or what's happening," Hoster told reporters Saturday. "We're asking for help from the public."
A fourth victim, 72-year-old Marshall Levine, was found dead Saturday in his office at a mental health counseling facility. Hoster said police are trying to see whether that shooting, which also took place in Scottsdale, is related to the others.
Police issued a sketch of the suspect -- based on witnesses to the Pitt killing -- but have not identified him. Police said they do not believe there is more than one shooter.
Evidence from Thursday's and Friday's scenes appears to connect the shootings, Hoster said.
The sergeant said he didn't have information that would indicate a serial killer is behind the shootings.
There is a $21,000 reward for information that leads authorities to the killer.