MOSCOW, Feb. 27 (UPI) -- Russian investigators said they were examining motives in the shooting death of opposition leader Boris Nemtsov that include the Ukraine conflict, Islamic extremism, or a plan to destabilize the country.
Investigative Committee spokesman Vladimir Markin said there are other motives to consider as well in the Friday night shooting, including Nemtsov's personal life and business ties.
"There are no doubts that the crime as well as the murder site were elaborately planned," Markin said Saturday.
Nemtsov was shot at least four times in the back with a Russian Makarov pistol from a passing white car. A woman he was walking with was not hurt.
Nemtsov, 55, was a former deputy prime minister under former Russian President Boris Yeltsin and was a sharp critic of President Vladimir Putin. As governor of the city of Nizhny Novgorod, Nemtsov earned the reputation as an economic reformer.
The shooting came hours after a radio interview in which he denounced Putin's "mad, aggressive" policies and a day before he was to lead an opposition protest against Russia's actions in the Ukraine crisis and the local economic crisis. The rally was canceled after the shooting, replaced with a demonstration to mourn his death.
U.S. President Barack Obama on Friday released a statement to "call upon the Russian government to conduct a prompt, impartial and transparent investigation into the circumstances of his murder and ensure that those responsible for this vicious killing are brought to justice."
In a telegram to Nemtsov's mother, published on the Kremlin's website Saturday, Putin denounced the shooting as "vile and cynical" and praised Nemtsov's work.
"Please accept my deepest condolences for the irreparable loss," Putin said. "Boris Nemtsov left his mark in the history of Russia, in politics and public life."
Just two weeks ago, Nemtsov voiced fears that his opposition to Putin would result in the president having him killed.
"You know, yes I do fear it. Not much, not as much as my mother does. If I feared him too strongly would it make sense to head an opposition party?" Nemtsov said.
At least six other Putin critics have been killed since his first term as president:
Anna Politkovskaya -- A Russian journalist and human-rights activist was shot and killed in 2006. She fiercely criticized Putin's war in Chechnya.
Alexander Litvinenko -- A former Russian spy poisoned with a toxic radioactive element in 2006. He accused the Russian government of terrorism and accused Putin of having a hand in Politkovskaya's death.
Sergei Magnitsky -- A Russian attorney found dead in a prison cell in 2009. He had exposed a massive tax fraud by the government in Russia by and was subsequently arrested.
Stanislav Markelov -- A Russian human-rights lawyer shot outside the Kremlin in 2009. He had represented Politkovskaya and others opposed to the Chechen war.
Natalia Estemirova -- A human-rights activist and documentary filmmaker abducted and executed in 2009. She was investigating human-rights violations by Russian troops in Chechnya.