There were three main theories about why REvil — which seemed to revel in the publicity and reaped huge ransoms, including $11 million from JBS — suddenly disappeared. One is that Mr. Biden ordered the United States Cyber Command, working with domestic law enforcement agencies, including the F.B.I., to bring the group’s sites down. Cyber Command proved last year that it could do just that, paralyzing a ransomware group it feared might turn its skills to freezing up voter registrations or other election data in the 2020 election. The second theory is that Mr. Putin ordered the group’s sites taken down. If so, that would be a gesture toward heeding Mr. Biden’s warning, which he had also conveyed, in more general terms, when the two leaders met on June 16 in Geneva. And it would come just a day or two before a U.S.-Russia working group on the issue, set up during the Geneva meeting, is supposed to hold a virtual meeting. A third theory is that REvil decided that the heat was too intense, and took the sites down itself to avoid becoming caught in the crossfire between the American and Russian presidents.