walking with didn't realize he had had been shot until the car was driving off. it was quick and professional and nobody has that training outside the government. >> in 2016, senator john mccain took it one step further. >> vladimir putin is a thug and a killer and a murderer and a kgb agent. he had boris nemtsov murdered in the shadow of the kremlin. >> this is personal insult. this is lousy behavior from a politician. >> that is putin's top aide and spokesman dmitry peskov. >> that's nonsense. it's nonsense. there's nothing to comment on. >> over the course of putin's time in power his regime has been accused of involvement in the deaths of many of its critics, including the journalist anna politskaya and the former kgb agent litvinenko. some russia experts say there are dozens more like them.