The city says fire crews are manually shutting down the sirens and a complete shutdown could take up to three hours.
City crews worked throughout the day Saturday to reactivate the system, and residents were warned to be patient in case the sirens came on again.
Dallas officials said they are collaborating with the Federal Emergency Management Agency to create a wireless alert system that would circulate messages to every cell phone in the area in the event of a real emergency.
City spokeswoman Sana Syed said the sirens started at 11:40 p.m. Friday and the system was completely shut off by 1:20 p.m. Saturday.
City officials asked residents not to call 911 during the malfunction and reported the longest wait time to get through to 911 was two minutes, the press release said.
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Technicians were trying to determine how to bring the system back online without the risk of it being hacked again, he said.
The sirens began sounding at 11:42 p.m. Friday. "We had no malfunction". The system is still down and the city is relying on other mass notification systems including reverse 911 and social media for emergencies. Officials urged residents not to call 911.
Reports are pouring in of tornado sirens sounding throughout the Dallas area, including downtown. "Making the necessary improvements is imperative for the safety of our citizens", Mayor Rawlings said.