Hackers suspected of being behind a massive ransomware attack have made a demand of $70 million in cryptocurrency in exchange for unlocking all of the affected systems.The demand appeared Sunday on a dark web site used by the Russia-linked REvil gang.The cyberattack Friday hit the systems of hundreds of companies and public agencies across the world.It involved a breach of the Miami-based software company Kaseya, which called the attack “sophisticated.”Kaseya said in a statement it had a detection tool available for customers to see if their systems were infiltrated, and that it hoped to begin bringing its data centers back online by the end of Monday.The FBI said REvil was responsible for a late May ransomware attack that shut down the operations of JBS, the world’s largest meat processing company.U.S. President Joe Biden on Saturday referred to his meeting with Russian President Vladimir Putin last month, suggesting the United States would hold Russia responsible if it were linked to the attack.“If it is, either with the knowledge of and/or a consequence of Russia, then I told Putin we will respond,” Biden told reporters.  This report includes information from the Associated Press and Reuters