BOSTON (AP) — Federal agencies warned that cybercriminals are unleashing a major ransomware assault against the U.S. healthcare system. Independent security experts say it has already hobbled at least four U.S. hospitals this month, and could potentially impact hundreds more.

In a joint alert Wednesday, the FBI and two federal agencies warned that they had “credible information of an increased and imminent cybercrime threat to U.S. hospitals and healthcare providers.” They said “malicious cyber actors” are targeting the sector with ransomware that could lead to “data theft and disruption of healthcare services.”

The attacks coincide with the U.S. presidential election, but do not appear to have any connection to it.

“We are experiencing the most significant cyber security threat we’ve ever seen in the United States,” Charles Carmakal, chief technical officer of the cybersecurity firm Mandiant, said in a statement. He’s concerned that the group may deploy malware to hundreds of hospitals over the next few weeks.

Alex Holden, CEO of Hold Security, which has been closely tracking the ransomware in question for more than a year, agreed that the unfolding offensive is unprecedented in magnitude for the U.S. Administrative problems caused by ransomware, which scrambles data into gibberish that can only be unlocked with software keys provided once targets pay up, could further stress hospitals burdened by a nationwide spike in COVID-19 cases.

The Russian-speaking cybercriminals suspected of the attacks use a strain of ransomware known as Ryuk, which is seeded through a network of zombie computers called Trickbot that Microsoft began trying to counter earlier in October. While the company has had considerable success knocking Trickbot command-and-control servers offline through legal action, analysts say criminals have still been finding ways to spread Ryuk.

The U.S. has seen a plague of ransomware over the past 18 months or so.

In September, a ransomware attack hobbled all 250 U.S. facilities of the hospital chain Universal Health Services, forcing doctors and nurses to rely on paper and pencil for record-keeping and slowing lab work. Employees described chaotic conditions impeding patient care. Also in September, the first known fatality related to ransomware occurred in Duesseldorf, Germany, when an IT system failure forced a critically ill patient to be routed to a hospital in another city.

Holden said he alerted federal law enforcement Friday after monitoring infection attempts at a number of hospitals, some of which may have beaten back infections. The FBI did not immediately respond to a request for comment.

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