DALLAS (AP) — Dallas officials concerned about hackers blame radio signal issues for more than 150 emergency outdoor sirens falsely sounding last weekend.

Dallas City Manager T.C. Broadnax on Monday said experts have determined the issue involved a radio system, not a computer issue. Broadnax says encryption has since been added.

Dallas officials initially blamed hackers for setting off the 156 sirens late Friday night and into early Saturday. City authorities on Monday did not indicate who they now believe activated the system by broadcasting a few tones, via radio or telephone signal.

The sirens had to be manually shut down. Emergency management officials on Monday said that the outdoor siren system was fully operational again.

City spokeswoman Sana Syed says additional safeguards are now in place. She didn't elaborate.