Supporters of the slain opposition leader have managed to communicate with Nemtsov's sources.
His description of the shooter does not match the investigation's main suspect.
Opposition figures have branded them "cowardly scum" and will press charges.
The human rights councillor said there were "reasonable ground" to suspect the men had been mistreated.
Russian media outlets have been providing increasingly odd scenarios behind the death of the opposition leader.
Facing an uncertain future, Russia's opposition leaders mourn the 'irreplaceable guy with brass balls'.