FBI Director Christopher Wray told the Wall Street Journal that the agency is investigating about 100 different types of ransomware, many of which trace back to actors in Russia.
In the interview published on Friday, Wray singled out Russia as harboring many of the known users of ransomware.
Each of the 100 different malicious software variants are responsible for multiple ransomware attacks in the United States, Wray told the newspaper.
The Kremlin on Friday said the FBI director's remark that Russia was a haven for hackers was "emotional", RIA reported.
Wray's remarks come days after a cyberattack disrupted much of meatpacker JBS SA's North American and Australian operations. The White House linked the attack to a Russia-based group.
Last month, the biggest U.S. gasoline pipeline Colonial Pipeline, was hit by a ransomware cyberattack, which crippled fuel delivery for several days in the U.S. Southeast. The FBI attributed the cyberattack to a group believed to be based in Russia or Eastern Europe.
The U.S. Department of Justice is elevating investigations of ransomware attacks to a similar priority as terrorism, a senior department official told Reuters on Thursday.
Ransom software works by encrypting victims' data; typically hackers will offer the victim a key in return for cryptocurrency payments that can run into the hundreds of thousands or even millions of dollars.