Kathmandu.  A chain of US hospitals has been hit by what is believed to be the largest cyber attack in history. The computer system of Universal Health Services of the United States has been attacked. The hospital functions in more than 400 locations across the country. CNBC quoted a source mentioning that computers of the hospital network had stopped working this weekend and hospitals had begun writing patient records with paper and pen.

Universal Health Services did not comment immediately but posted a statement on its website later stating that its company network was now offline due to technical problems. An anonymous hospital official, unauthorized for media communication, stated the attack appears to be ransomware.

Ransomware is a type of malicious software that spreads over a computer network, encrypting files and demands money for a key to decrypt them. Although this is a common tactic used by hackers, attacks of this magnitude on medical services aren’t usual. A patient had to be moved to another hospital following a ransomware attack on a German hospital recently, where she died, possibly the first death pertaining to ransomware.

The hackers have been installing ransomware on the hospital system since this weekend, aware of lesser technical staff at the time. According to a nurse at a North Dakota hospital, the computers were slow at first and have been completely shut down since Sunday morning. Another nurse from Arizona also mentioned the computers shutting down on their own.