Doug Stanglin

USA TODAY

Dallas city officials said Saturday that a hacker is to blame for setting off all the city's 156 emergency outdoor sirens, which wailed for an hour and half overnight.

Rocky Vaz, director of the city's Office of Emergency Management, said engineers determined an unidentified hacker somewhere in the Dallas area was responsible, but has not been tracked down.

The hacker tricked the system to send repeat signals activating each siren 60 times during the night, Vaz said. The sirens started sounding at 11:42 p.m. Friday and continued until 1:17 a.m. Saturday.

The blaring sirens, used primarily to warn of tornadoes and other severe weather, prompted anxious residents to call 911, clogging up that system. At one point, 911 calls were backed up for six minutes instead of the normal wait time of 10 seconds.

Dallas spokeswoman Sana Syed said the 911 system was bombarded with over 4,400 calls between 11:30 p.m. and 3:00 a.m., double the number for a normal, eight-hour period overnight.

Mayor Mike Rawlings said in a statement that the hack was an attack on Dallas' emergency notification system, and that the city will "find and prosecute whomever is responsible."

"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. "It's a costly proposition, which is why every dollar of taxpayer money must be spent with critical needs such as this in mind. Making the necessary improvements is imperative for the safety of our citizens."

Vaz said he expects the emergency siren system to be back in operation by Monday afternoon.

Locating the hacker, however, "is going to be a very long process if we do find out who actually did it," Vaz said. The Federal Communication Commission is helping in the investigation, he added.