When Boris Nemtsov was shot dead as he walked across a bridge next to the Kremlin, it took 11 minutes before a police car arrived at the scene, according to the time stamp on closed-circuit television footage.

From the moment the 55-year-old former deputy prime minister was shot late on the night of Feb. 27, associates of the Kremlin critic have been asking why the police took so long to get there, and how someone could fire six shots at him in such a heavily monitored location and then get away.

Nemtsov is the most high-profile opposition figure who has been killed during President Vladimir Putin's 15-year rule. His shooting prompted accusations from Putin opponents and Western states that democratic freedoms in Russia were under attack.