A computer hack set off all the emergency sirens in Dallas for about 90 minutes last weekend in one of the largest known breaches of a siren warning system.

Dallas' 156 sirens, normally used to warn of tornadoes and other dangerous weather, were triggered at 11:42 p.m. CDT on Friday, reported Reuters.

Engineers had to manually shut down the sirens' radio system and repeaters, Reuters said. The breach in the city of 1.6 million people was believed to have originated in the area, said the Reuters report.

The hack is being investigated by system engineers and the Federal Communications Commission has been contacted, but police have not been involved, it said.