Websites for a Russian-linked ransomware gang blamed for attacks on hundreds of businesses worldwide have gone offline.
Monitors say a payment website and a blog run by the REvil group became suddenly unreachable on Tuesday.
The reason behind the disappearance is unknown, but has sparked speculation that the group may have been targeted deliberately by authorities.
It comes amid growing pressure between the US and Russia over cyber-crime, reports the BBC.
US President Joe Biden said he raised the issue with Vlamidir Putin during a phone call on Friday, after discussing the subject during a summit with the Russian president in Geneva last month.
Mr Biden told reporters that he had "made it very clear to him...we expect them to act" on information and also hinted the US could take direct digital retaliation on servers used for intrusions.
The timing of Tuesday's outage has sparked speculation that either the US or Russian officials may have taken action against REvil - though officials have so far declined to comment and cyber experts say sudden disappearances of groups are not necessarily uncommon.
The development comes after a series of high-profile ransomware attacks which have hit major US businesses this year.
The FBI accused REvil - also known as Sodinokibi - of being behind a ransomware attack on the world's largest meat processing company JBS last month.
The group is considered prolific and last week demanded a huge bitcoin ransom for an attack which targeted IT firm Kaseya and hundreds more businesses worldwide.
