Kenyan police accuse a human rights group of being reckless for alleging they carried out nearly 500 execution-style killings in Nairobi.
Police made hundreds of arrests of Mungiki members in June
The Kenya National Commission on Human Rights (KNCHR) made the claim after investigating the disappearance of hundreds of men from the Mungiki sect.
Police carried out a major operation against the outlawed criminal gang in June after a series of grisly murders.
But Kenya's police chief said it is wrong to jump to premature conclusions.
"All investigations in the world, anywhere in the world, take time to conclude," Kenyan Police Commissioner Mohammed Hussein Ali told BBC News.
"These are not simple matters. It is wrong therefore for anyone to jump to the conclusion that the police, the local community, or indeed anyone else, is guilty," he said.
The Mungiki terrorised parts of the capital city, Nairobi, and central Kenya earlier this year, demanding protection fees from public transport operators, slum dwellers and other businessmen. Many who refused were brutally murdered.
Banned in 2002, the sect's members are thought to be militants from Kenya's biggest ethnic group, the Kikuyu.
'Classic execution signs'
The dossier published by the KNCHR on Monday alleges that police were behind the execution-style killings of nearly 500 men during the crackdown on the Mungiki between June and October this year.
More than 450 of the bodies were found at Nairobi's City Mortuary after relatives reported loved ones missing following a major police operation against the Mungiki sect, the KNHCR said. A further 11 bodies were found in Machakos and another 11 in Naivasha.
Banned in 2002
Thought to be ethnic Kikuyu militants
Mungiki means multitude in Kikuyu
Inspired by the Mau Mau rebellion of the 1950s
Claim to have more than 1m followers
Promote female circumcision and oath-taking
Believed to be linked to high-profile politicians
Control public transport routes, demanding levies
Blamed for revenge murders in the central region
"Almost all the cadavers bear classic execution signs of a bullet behind the head exiting through the forehead," the KNHCR said.
The findings "lead to the inescapable conclusion that the police could be complicit in the killing", it added.
"The KNCHR is also extremely concerned that the emerging pattern points to possible complicity of state security agents in the disappearance of persons."
Observers at the time also said the police campaign had led to the deaths of several innocent civilians.
'Jumping to conclusions'
The director of the human rights group, Maina Kiai, told the BBC he was convinced Kenyan authorities were hiding the truth.
"Five hundred people dying in the space of four months and nobody knowing about it seems to suggest that there is officialdom trying to cover up and being able to," he told the World Today.
"The manner of the executions was also very, very telling, because almost everybody was shot at the back of the head. Thirdly, we found out that the police had not carried out any investigations."
Mr Kiai said the KNCHR had no direct evidence of police involvement in the killings, but insisted it was still their duty to find out who was responsible.
"I think we in this country and in the world need to ask questions of this government and of the police, because 500 people dead in four months is not usual anywhere in the world unless there is conflict," he added.