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.” The alert said malicious groups are targeting the sector with attacks that produce “data theft and disruption of healthcare services.”

The cyberattacks involve ransomware, which scrambles data into gibberish that can only be unlocked with software keys provided once targets pay up. Independent security experts say it has already hobbled at least five U.S. hospitals this week, and could potentially impact hundreds more.

The offensive by a Russian-speaking criminal gang coincides with the U.S. presidential election, although there is no immediate indication they were motivated by anything but profit. “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.