Cybercriminals attacked the computer systems of a California-based health care provider causing emergency rooms in multiple states to close and ambulance services to be redirected.

The ransomware attack happened at Prospect Medical Holdings of Los Angeles, which has hospitals and clinics in Connecticut, Pennsylvania, Rhode Island and Texas. Prospect Medical is investigating how the breach happened and is working on resolving the issue, the company said in a statement Friday…

Officials with the Pennsylvania Association of Staff Nurses and Allied Professionals, the nurses’ union at Crozer-Chester Medical System in Springfield, say the hospital has reverted to a paper system because most of the computers are offline, CBS News reported. The computers are unlikely to be back online until next week, according to the labor group.