Dallas is experiencing IT and police communication outages following a ransomware attack on municipal systems.

City officials said in a statement Wednesday that hackers had compromised “a number of servers” and that they were working with vendors to try to control the spread of malware. With a ransomware attack, hackers lock up victims’ data or knock services offline, then demand an extortion payment. 

The Dallas police department’s website was inaccessible when Bloomberg News tried to visit the page Thursday. The hack also affected the police dispatch system and resulted in the closure of local courts on Wednesday, TechCrunch reported.