Emergency sirens around Dallas, Texas, activated late on Friday night, waking residents across the city for over an hour, and prompting a flood of calls to the city’s 911 center. Officials from the city’s emergency management office have confirmed that there was no emergency, and that the system was breached by hackers.

Sana Syed, the director of the city’s Public Information Office told reporters that the city’s 156 sirens were activated, and that “it does appear at this time that it was a hack, and we do believe that this came from the Dallas area.” City officials did not disclose any further details about the nature of the breach. According to Syed, the system is now back online and operational.

Rocky Vaz, the director of the city’s Office of Emergency Management, explained that the city’s sirens were activated on Friday night, and went through 15, 90-second cycles before being shut down. When city OEM workers determined that there wasn’t an emergency, had trouble turning off the sirens. Syed noted that at several points, they thought that the system had been deactivated, only to have the sirens turn on again. Eventually, city officials were forced to essentially unplug the entire system to deactivate it completely. After investigating, they were able to locate “one area where we think [the attackers] were able to get into our system and activate all the sirens,” says Vaz.

The city is now looking into ways to protect the entire system from being attacked again. The breach is believed to be one of the largest of its kind, according to Vaz. Dallas city mayor Mike Rawlings told The Dallas Morning News that the incident was “yet another serious example of the need for us to upgrade and better safeguard our city's technology infrastructure,” and that the city is working to “identify and prosecute those responsible.”