the mystery deepness into the assassination of one of russia president vladimir putin s top critics. boris nemtsov gunned down before he was set to lead a rally. did he hold damning evidence against the kremlin? let s together to matthew chance in moscow. reporter: you can see, alisyn, a sea of flowers that was been made this tribute for boris nemtsov. in the shadow of the cripple len ichlt you can still see days after the killing there s a trickle of well-wishers that are coming by paying their respects to this say sass that theed political opposition leader. all this a day after one of the biggest rallies that the russian capital has seen for years.