MOSCOW (REUTERS) - Russia's Federal Security Service and tax inspectors on Thursday (April 14) searched the offices of Mikhail Prokhorov, a tycoon whose newspaper has published revelations about people close to Vladimir Putin, four sources close to his businesses said.

The searches were being conducted at Onexim Group, which manages the assets of 50-year-old Prokhorov, who according to Forbes magazine is Russia's 14th richest man with wealth of US$7.6 billion (S$10 billion). The sources said the reason for the searches was not clear.

A Reuters reporter outside the office building said the main entrance had been locked. A man who came outside to smoke said some "serious guys" had arrived in the building and were restricting access to some areas of the office.

The Federal Security Service could not immediately be reached for comment. A spokesman for the tax service said she did not yet have any information.

An Onexim spokesman told Reuters the tax authorities were conducting investigative activities at the company. He said Onexim operates in full compliance with the law and is ready to cooperate with the authorities.

Prokhorov owns stakes in potash producer Uralkali, aluminium producer Rusal and a controlling stake in RBC media, publisher of a newspaper which has reported on Putin's family and friends and their commercial interests.

He is also the owner of N.B.A franchise the New Jersey Nets.

One of the sources said that searches were also being conducted in Prokhorov's investment firm Renaissance Capital. A spokesman for the firm was not available for comment.

A representative of power generating company Quadra , also controlled by Prokhorov, said that there were some checks underway in his office, probably with the Tax Service involved.

Uralkali and Rusal were not affected by the searches, their representatives said.