The informal group of monitors includes opposition members, former law-enforcement officials, and other public figures.
A separate statement from the opposition group For a Just Kazakhstan raised similar doubts and called for the creation of a state commission to investigate the case.
Its statement came one day after Interior Minister Baurzhan Mukhamedjanov said the head of the Senate administration, Erzhan Utembaev, commissioned the murder of Altynbek Sarsenbaev, the co-chairman of the Naghyz Ak Zhol (True Bright Path) party.
In a further development, another senior official of the Kazakh security service, the KNB, stepped down on 28 February. Seytzhan Koybakov headed the KNB's elite Arystan (Lion) unit.
His move follows the arrest of several Arystan members and their subsequent confession to the crime. The KNB's chairman, Nartay Dutbaev, resigned on 22 February.
Sarsenbaev's body was found on the outskirts of Almaty on 13 February, along with the bodies of his bodyguard and a driver.
Opposition figures have accused government authorities of playing a role in the killing.