A hacker set off all 156 of Dallas's emergency sirens Friday night.
"It does appear at this time it was a hack, and it does appear this came from the Dallas area," Sana Syed, the city's managing director of public information, said at a Saturday news conference.
The alarms sounded at about 11:40 p.m. and were turned off by 1:20 a.m. Saturday, but not before officials had to shut down Dallas's emergency system.
The FBI sought to assure residents - thousands of whom called 911 - by tweeting that the emergency sirens were malfunctioning and noting there was no bad weather or active emergency afoot. Authorities are investigating.