Authorities raided Nginx's Moscow office based on a copyright infingement claim by Russian oligarch Alexander Mamut’s Cyprus-registered investment vehicle Lynwood, Forbes Russia cited an unnamed source at the web server as saying. Mamut became part owner of Russia’s 1990s-era search giant Rambler with fellow oligarch Vladimir Potanin in 2013 and bought out Potanin’s stake three years later.

Russian police raided the Moscow offices of a popular U.S.-owned web server over a Russian internet giant’s ownership claim of its source code, Forbes Russia reported Thursday.

“We found that Rambler Internet Holding’s exclusive right to the Nginx web server has been violated by the actions of third parties,” Rambler’s spokesperson told Forbes.

“In this regard, Rambler Internet Holding ceded the rights to bring claims and lawsuits linked to rights violations toward Nginx to Lynwood Investments CY Ltd,” it continued.

Authorities estimate Rambler’s losses from the alleged copyright infringement at 51.4 million rubles ($820,000), according to a copy of a criminal case cited by Forbes and other news outlets.

Mamut was spotted leaving a Sberbank event midday Thursday not long after the Nginx raid began, Forbes cited a witness as saying. Sberbank, Russia’s largest lender, bought 46.5% of Rambler Group in April.

Authorities briefly detained Nginx’s developer and former Rambler system administrator Igor Sysoyev along with co-founder Maxim Konovalov in the raid, Forbes reported.

Sysoyev has said that he developed Nginx in 2002 outside of his work for Rambler and that programming was not in his job description.

Rambler was aware of Sysoyev’s independent open-source project when it hired him in 2000, its former executive Igor Ashmanov who made the hiring told Forbes.

Nginx is one of the world’s most popular open-source web servers and is used by nearly 38% of all websites. U.S. developer F5 Servers bought Nginx for $670 million in March 2019.

An earlier version of this article described Rambler as a "search engine giant." It has been changed to "internet giant."