Hackers took control over the 156 sirens in Dallas this weekend, triggering false alarms on the system used to alert residents to take shelter from inclement weather, until officials deactivated the system early Saturday morning.

The person or people responsible were able to hack into a part of the system that was communicating with all 156 of the city’s sirens, said Rocky Vaz, who heads the city’s office of emergency management, at a news conference.

Technicians were trying to determine how to bring the system back online without the risk of it being hacked again, he said. In the meantime, the public has access to other alert systems. Vaz said he hoped to have the siren system back up by Sunday.

The sirens began sounding at 11.42pm Friday. Officials deactivated the entire system by 1.17am Saturday. Vaz said the sirens went through about 15 cycles of a 90-second siren activation.

“We shut it down as quickly as we could, taking into consideration all of the precautions and protocols we had to take to make sure that we were not compromising our 156-siren system,” he said.

City spokeswoman Sana Syed said officials believe the hack came from the Dallas area. Vaz said city officials have asked the Federal Communications Commission for help in determining who was behind the hack. He said police were not involved yet.

“We can’t talk a whole lot about the hack itself, because obviously we don’t want this to happen again,” Syed said. “We cannot discuss the details of how this was done, but we do believe that the hack came from the Dallas area.”

Syed said that there was a surge in calls to 911 during the time the sirens were sounding. She said that they had about 4,400 calls from about 11.30pm Friday to about 3am Saturday. She said that usually from 11pm to 7am they get about half that number of calls. The largest surge came from midnight to 12.15am, when about 800 calls were received. The longest wait time was six minutes.

Mayor Mike Rawlings wrote in a Facebook post that the hack was “an attack on our emergency notification system” and that the city needed to upgrade its systems. “We will work to identify and prosecute those responsible,” he wrote.