"For security reasons, we can not discuss the details of how this was done, but we do believe that the hack came from the Dallas area".
The city was forced to manually shut down the siren system, which is typically only used during weather emergencies such as a tornado threat.
Dallas' 156 sirens, normally used to warn of tornadoes and other unsafe weather, were triggered at 11:42 p.m. CDT on Friday.
Syed said that they had about 4,400 calls from about 11:30 p.m. Friday to about 3 a.m. Saturday.
Locating the hacker, however, "is going to be a very long process if we do find out who actually did it", Vaz said. Until the sirens are running, Dallas will rely on local media, emergency 911 phone calls, and a federal radio alert system, Vaz said.
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Surrounding cities also reported hearing sirens, but officials confirmed only the sirens in Dallas were triggered. A city spokesman originally said a system malfunction had caused the sirens to activate. "We had no malfunction".
Vaz said his office had to carefully shut down the system to make sure they didn't inadvertently damage it and now have to wait for a thorough investigation to make sure it can't be hacked again. They won't know the health of the sirens until the system is reactivated.
Dallas Mayor Mike Rawlings promised the perpetrator would be found and prosecuted.
Residents are asked to be patient and to avoid calling 911 or 311 as the sirens may sound again during reactivation.
"This is yet another serious example of the need for us to upgrade and better safeguard our city's technology infrastructure", he posted on Facebook.