A human rights panel on Monday implicated the Kenyan police in the execution-style deaths of nearly 500 men, but the claims were dismissed out of hand by the security services.
The state-run Kenya National Commission on Human Rights (KNCHR) linked the slayings to a war between police and a violent street gang accused of a string of beheadings and fatal shootings earlier this year.
The KNCHR said the victims were executed by a single bullet between June and last month. More than 450 bodies were found in the capital's City Mortuary, 11 in the eastern town of Machakos, and another 11 in the Rift Valley town of Naivasha, the KNCHR said in a report on alleged executions and disappearances.
"Almost all the cadavers bear classic execution signs of a bullet behind the head exiting through the forehead," it said.
The findings "lead to the inescapable conclusion that the police could be complicit in the killing. The KNCHR is also extremely concerned that the emerging pattern points to possible complicity of state security agents in the disappearance of persons," it said.
KNCHR chief Maina Kiai called on police to explain how hundreds of bodies ended up in the mortuaries recorded on police registers, yet the force had flatly rejected any involvement in the deaths.
"There is need for a sense of accountability in our security forces ... Killing about 500 people without due process is a crime against humanity," Kiai told a press conference.
"We want police to tell us how those 500 people ended up in the mortuary between June and October. The burden of proof lies with the police," he said.
"The obvious question to ask is, if the police are themselves not responsible, why have they been unwilling or unable to investigate and curb the killings?" Kiai said.
He said police had been lax in following up tips passed on by residents, and refused to collect some of the bodies after their presence had been reported, leaving them to hyenas and other wild animals.
"Which citizen or organized criminal group would have the wherewithal and courage to ferry corpses for dumping on our roads, which are mounted with police checks after every few kilometers on a 24 hours basis?" Kiai said.
Maina said that during investigations his panel had encountered "at all layers of the police hierarchy ... stone walling, disinterest and outright denial of any knowledge on the killing and dumping of the bodies."
Police angrily dismissed the execution accusations as fodder for "horror movies."
"That is a very irresponsible statement. I do not know where they got the figures," national police spokesman Eric Kiraithe told independent NTV television.
"This is a very weird piece of imagination that would have passed as horror movie. I would like Maina Kiai to produce a post-mortem report from an authentic doctor declaring the cause of death," Kiraithe said.
"We invite anybody who have information about any killings to come to us. We will investigate it," he said.
The police declared months ago a war on the politically linked Mungiki religious gang, which was banned in 2002 after deadly slum warfare that claimed the lives of dozens of people.
Mungiki has been linked in recent years to extortion, murder and political violence. Its members also promote traditional Kikuyu practices, including female genital mutilation. Since March, the gang has been accused of murdering at least 43 people -- beheading several of their victims -- mainly in Nairobi slums and central Kenya.