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Authorities do not know if the Dallas siren hack was the handiwork of a lone hacker or a group of hackers iStock

Unidentified hackers set off all the 156 emergency sirens in Dallas in one of the largest known breaches of a siren warning system waking up city residents in panic.

The sirens most often used to warn about extreme weather conditions like tornadoes were triggered at 11:42 pm CDT on 7 April. It continued for nearly two hours until engineers manually shut down the sirens' radio system and repeaters. Within 15 minutes of the sirens going off, emergency hotline numbers received around 800 panic calls from residents.

Emergency Management Director Rocky Vaz told Reuters: "We can tell you with a good deal of confidence that this was somebody outside of our system that got in there and activated our sirens."

Although it is not known if a single hacker or a group of hackers targeted the system, a city spokesperson said it was believed to have originated within the city itself.

Describing the incident as a very rare one, Vaz said engineers had begun restarting the system. Early Sunday morning Sana Syed, the city's public information officer, tweeted saying: "The outdoor warning siren system now operational. More safeguards being put in place this wknd. More info tomorrow. "

The Federal Communications Commission (FCC) has reportedly been informed of the hack but police have not been involved so far.