In a secret chat room run by a group of Russian-affiliated cybercriminals, a hacker expressed excitement about a plan to attack and disable more than 400 U.S. hospitals. “There will be panic,” the hacker wrote, in Russian. The hacking enterprise, called the Trickbot Group by federal prosecutors, and its affiliates had already collected hundreds of millions of dollars by shutting down emergency rooms, city governments and public schools since 2018. Security researchers and U.S. officials say the internal conversations amount to the most complete and candid public look yet at the operations of a criminal ransomware enterprise. A malware developer...