MOSCOW (AP) — Russia's main domestic security agency has searched the offices of a company controlled by the billionaire owner of U.S. basketball team Brooklyn Nets, Mikhail Prokhorov.

The Federal Security Service, the main KGB successor agency, said Friday the searches of ONEXIM group were part of a probe into alleged tax evasion. It added that the investigation is continuing.

Some Russian media have speculated that the searches could have been aimed at pressuring Prokhorov into surrendering control over his media assets. They include RBC, an online newspaper that has published reports critical of the Kremlin, including a detailed account of the so-called Panama Papers detailing offshore assets allegedly owned by top Russian businessmen and officials.

The searches on Thursday went on just as President Vladimir Putin held his annual call-in TV show. Putin's spokesman Dmitry Peskov strongly denied any link between the searches and Prokhorov's media assets.

The 50-year-old Prokhorov, who is worth $8.2 billion according to Forbes, founded a liberal party and challenged Putin in the 2012 presidential election, winning 8 percent of the vote. He hasn't commented on the latest searches.

Russian authorities have frequently used accusations of tax evasion to crack down on unruly tycoons. Mikhail Khodorkovsky, once Russia's richest man, spent 10 years in prison on charges of tax evasion and embezzlement which were widely seen as a political vendetta for challenging Putin's power.