The city says the hackers are persistent and not giving up.
It's the first time that the hackers were able to overwhelm the city's system.
Leaders say over the past week they've seen an increase in activity from unknown people trying to access the website and internet based services.
They say they were able to stop the hackers before they accessed information.
"The city's internet access was down for 45 minutes," said Randall Smith, the city's Chief Security Officer.
The City of Phoenix is investigating an attack by hackers who blocked the city's internet systems Saturday.
"At this point we're unclear about any kind of motive or who's attacking us, we don't know," said Smith.
Smith confirmed the attack after fOX 10 obtained internal e-mails, one of which showed the system was under attack for days.
A deputy city manager wrote; "this appears to be a coordinated, denial of service attack."
Then over the weekend a city police memo said because of the hacking officers couldn't access computers in their cars allowing them to access driver's license, license plates, and criminal history information.
"There's always concerns if something happens to our public safety systems the primary communication is the radio with fire and police so they always had communication we are looking closely at that and proactively looking at solutions to minimize future outages such as this," he said.
Smith emphasized the hackers aren't targeting Phoenix Police but are just trying to find any way into the system.
"The purpose of the attack is to try to overwhelm our perimeter defenses and firewalls, the goal of the attacker is to try to gain access into our network generally to obtain personal identified information for financial gain," said Smith.
Smith said the hackers did not get any information or data, but he's trying to stay one step ahead.
"We're starting to see more probes as we'll call them from these internet hackers activists, we are actively working with our technology partners and law enforcement to proactively address the situation in looking at our solutions and procedures in place," he said.
So the police computers that were down, the MDC's for police were back up and running after that 45-minute period.
City leaders say they've contacted the FBI. Smith says many private companies and governments are facing these kinds of cyber attacks.