Duritskaya, witness to Nemtsov's death, in custody

MOSCOW, March 2 (UPI) -- Anna Duritskaya, the key witness in the Moscow shooting death of opposition leader Boris Nemtsov, is under police guard and unable to leave Russia.

Duritskaya, 23, a Ukrainian, accompanied Nemtsov, 55, at dinner Friday, then walked with him across a bridge when he was shot four times in the back and died. Duritskaya was not injured, but told the Russian television channel Drodh she was unable to identify the shooters or the vehicle they used to escape.

Nemtsov was Russia's deputy prime minister in the 1990s under Boris Yeltsin, but fell out of favor in the administration of current Russian President Vladimir Putin. He became a Putin opponent on a number of issues, most prominently Russia's involvement in the conflict in Ukraine.

Duritskaya said she was eager to return to Ukraine, but was being held by Russian investigators in the Moscow apartment of friends. "I've told them everything possible, and I don't know why I am still on Russian territory, as I want to go back to my mum, who is ill and ... in a very difficult psychological state," she told Drodh. "For three days, I've been driven around in police cars with guards to the investigative committee, and questioned. And nobody has told me when I'll be let go and why I am being held here."

Vladimir Markin, a spokesman for the Investigating Committee of Russia, the country's main federal investigation agency which answers directly to Putin, suggested Nemtsov was killed over personal issues, arguments within Russia's opposition leadership or Islamic terrorism; Markin pointedly avoided mentioning any idea of a government assassination.

Tributes to Nemtsov have been plentiful, and both Putin and Russian Foreign Minister Sergei Lavrov said the shooting would be thoroughly investigated.

Ten Putin opponents have died violently since 2003. While most, like investigative journalist Anna Polikovskaya and liberal political leader Sergey Yushenkov, have been shot to death, former Russian spy Alexander Litvinenko died in 2006 after drinking polonium-laced tea in London, and lawyer Sergei Magnitsky died in 2009 of injuries sustained while in police custody.