MOSCOW, June 18 (RIA Novosti) - The son of the vice president of Russian oil giant Rosneft was freed in the early hours of Thursday, some three months after being kidnapped in Moscow, a senior investigator said.

Mikhail Stavsky's son, who is also named Mikhail Stavsky, is reported to have been forced into a BMW on April 13 outside the Russian State Oil and Gas Institute.

"The investigation is continuing. Those linked to the crime are being identified," chief Moscow investigator Anatoly Bagmet said, giving no further details.

The case has been shrouded in mystery, with few comments available from police or investigators. Deputy Prime Minister Igor Sechin heads the Rosneft board of directors.

Media earlier cited investigators as saying they were looking into different motives, including ransom and attempts to put pressure on the victim or his family.