An anti-Mungiki operation in which 500 youths were said to have been killed was secretly carried out by a team of police officers sanctioned by their seniors.
The Kenya National Commission on Human Rights (KNCHR) filmed confessions by four policemen attached to the hit squad to support the claims it released on Tuesday.
It reveals the hit squad comprised of 14 officers who were sometimes reinforced by colleagues from other units. But one of the officers who confessed was tracked down three-and-a-half months later and shot dead, allegedly by his colleagues.
KNCHR on Tuesday released a video containing revelations by the officer, Mr Bernard Kiriinya, but confessions by the other three have been kept secret for fear they too, could be eliminated.
Like Mr Kiriinya before he was killed, they are sheltered in safe houses in and outside Kenya.
The confessions formed part of the KNCHR report on extra-judicial killings released last year.
UN Special Rapporteur on extra-judicial killings, Prof Philip Alston is in the country to investigate the claims.
In the video lasting 80 minutes, Mr Kiriinya said he witnessed the killings of 58 suspected Mungiki members.
Among the victims is Kimani Ruo, the Rift Valley province Mungiki co-ordinator who disappeared after being freed by a Nairobi court in June 2007.
The officer also claimed the wife of jailed Mungiki leader Maina Njenga was killed by police officers who robbed her of Sh5 million.
According to the confession, the killers were drawn from the Special Crime Prevention Unit (SCPU), the Nairobi provincial headquarters and CID.
Mr Kiriinya, who had been attached to the SCPU since 2005 as a driver, said the hit squads could detain suspects at any police station.
And for a “job well done,” officers were irregularly promoted. An officer whose name we cannot disclose for legal reasons was promoted two ranks higher than the one he held.
Constable Kiriinya was shot dead on October 16 last year a few metres from his house.
“The morning before, he was visited by a person whose car’s registration number was noted in the visitor’s book. The vehicle belongs to an officer based at the SCPU,” said KNCHR vice-chairman Hassan Omar Hassan.
Police spokesman Eric Kiraithe said KNCHR’s claims as “callous and irresponsible.” Mr Kiraithe said the revelations were stage-managed to coincide with the UN Special Rapporteur’s visit.