WASHINGTON—The Russia-based criminal syndicate behind a devastating series of recent ransomware attacks was offline on Tuesday, but cybersecurity experts said that it was premature to speculate why and that there was no indication of a law enforcement takedown. REvil’s dark web data-leak site and ransom-negotiating portals were both unreachable, cybersecurity researchers said. The group was responsible for the Memorial Day ransomware attack on the meat processor JBS and the supply-chain attack this month targeting the software company Kaseya that crippled well over 1,000 businesses globally. President Joe Biden told Russian President Vladimir Putin on a call Friday that he needed to rein in attacks from Russia-based groups and warned that the United States had the right to defend its people and critical infrastructure from attacks. But there were no immediate or public signs that the government had anything to do with REvil appearing offline. It was also possible that the …