What appears to be a major ransomware attack has hit the computers of Universal Health Services, one of largest hospital chains. This affects hundreds of locations.

A network outage at Universal Health Services hospitals and clinical facilities across the country began overnight Sunday, forcing employees onto paper backup systems.

Cybersecurity experts say the incident is likely a ransomware attack, in which hackers lock up computer networks, often steal information and demand payment to restore access.

There were anecdotal reports of disrupted operations at various hospitals, from Wi-Fi-based patient monitors not functioning, to emergency room wait times growing exponentially and canceled surgeries.

Universal Health Services released a short statement saying its network is offline due to an IT security issue and that no patient or employee data appears to be compromised.