A British investor who made millions in Russia before his lawyer was imprisoned and died is calling for an international investigation into the killing of Russian opposition figure Boris Nemtsov.

U.S.-born William Browder is suggesting a probe led by the United Nations, the Council of Europe, or the Organization for Security and Cooperation in Europe.

Vladimir Putin's spokesman has called the killing a "provocation," and Russian authorities are investigating. But many accuse the Kremlin of involvement.

Browder says "we're never going to get to the bottom of this if it is left in the hands of the people who are at the top of the suspect list."

Nemtsov supported the case of whistleblower lawyer Sergei Magnitsky, who worked with Browder's investment fund and died in prison at age 37.