The death toll in a spree of Maryvale shootings has reached six, after one of the victims shot last month died in the hospital this week.

Police aren’t ready to use the words “serial killer,” but one criminal expert said he will.

The six people were shot in four separate incidents that started in April.

See a map of the shooting locations below.

“I think what we can say relatively confidently is that by definition we have a person who is committing serial offenses, or who is engaged and serial killing,” said Steven Pitt, a nationally-renowned forensic psychiatrist based in Scottsdale.

Pitt is an expert in criminal behavior who has consulted on high profile cases across the country, including the JonBenet Ramsey murder. He’s also well-versed in the psychology of spree killing.

“That evidence starts with the victimology and really trying to understand each scene and each victim and whether or not there's a relationship between these victims; which as I understand there is not,” said Pitt.

Three of the Maryvale victims were shot at the same scene while they were sitting in a car together. The other victims were each by themselves. All were killed on weekend nights and all were shot in front of residential homes within a couple miles of each other.

The first shooting was in April and the others were in June.

“[This] could be a very sophisticated offender who's lying low right now because the heat is on. We just don't know. It could be that the offender moved on. We just don't know.”

Pitt cautioned that serial murders don't necessarily mean just one killer is responsible.

Silent Witness is offering $16,000 for anybody who can help crack the case.