PHOENIX >> One year after a suspected serial killer set off on a wave of shooting attacks in Phoenix that killed seven people and wounded two, police say they have experienced a significant slowdown of information provided by the public that could help detectives solve the case.

The high volume of tips that police received last year during the four-month period that the killer was active in two neighborhoods has dropped dramatically and no new physical evidence has turned up since the last known attack eight months ago, said police spokesman Sgt. John Howard.

Despite the lack of new information that could lead to an arrest, police remain committed to finding the killer dubbed the Serial Street Shooter suspect and believe that the case will be cracked by a tip from someone who knows the attacker, he said.

“Many of the investigators have told me this is one of the most difficult cases they have investigated,” said Phoenix police spokesman Sgt. Jonathan Howard.

Friday marked the one-year anniversary since the first attack linked to a single suspect.

The first shooting happened on March 17, 2016, when a Nissan drove past two teenagers, pulled a U-turn and a man inside the vehicle opened fire, hitting a 16-year-old boy in the arm, abdomen and hip. The teen survived the attack. In the most recent attack, on July 11, a 21-year-old man and his 4-year-old nephew escaped injury after the gunman shot at a vehicle they were sitting in.

Police have the victims were attacked as they stood outside their homes or sat in vehicles after dark. They were fired upon by someone who was sitting in a car or had just stepped out of a vehicle. All but one of the killings took place in the city’s Maryvale section, a predominantly Hispanic neighborhood Phoenix’s west side.

Investigators believe the crimes were carried out by a lanky Hispanic man in his early 20s, but they are leaving open the possibility that someone else may have participated in the attacks. They don’t believe the attacks are racially motivated, though no motive has been established.

The victims include a 21-year-old man whose girlfriend was pregnant with their son and a 12-year-girl who was shot to death along with her mother and a friend of the woman.

It’s not unusual for investigations into serial killers to stall, experts said.

“Sometimes these dry spells go on for years, but people shouldn’t mistake that for cases being dormant,” said Mary Ellen O’Toole, a former FBI profiler who directs George Mason University’s forensic science program.

Serial killers typically get caught through a tip from a member of the public or through the re-examination of evidence that had been looked at earlier, she said.

Investigators in the Phoenix case believe the case will be solved by a tip from a person who knows about the killer’s activities, perhaps a friend or relative who feels guilty about knowing about the attacks.

“The problem is that most family members and friends will not snitch unless they are absolutely sure as to the identity of the killer,” said Jack Levin, a professor emeritus of sociology and criminology at Northeastern University in Boston and author of several books on serial killings.

And it’s not unusual for serial killers to disappear for a period after they kill, said Mike Rustigan, a professor emeritus of criminal justice at San Jose State University who has studied serial killers.

The killer may have gone “into a cooling-off period to lay low because he doesn’t want to get caught,” Rustigan said

Investigators in Phoenix have continued to analyze physical evidence recovered from the shooting scenes, sought out neighbors who might have security camera footage of the shootings and worked with community groups to encourage tipsters to come forward.

After the number of tips about the killer dropped sharply

Once the number of tips declined sharply, tips that had already been examined were handed off to other investigators to be re-examined in hopes that a fresh set of eyes might turn up something new. Police also released reports and 911 recordings from the attacks to news organizations to try to keep the public aware of the case.

“If someone has information, they can share that with us today, and it can make a difference in the case this afternoon,” Howard said.