The box included a note reading, "Kavuludi you are next," which detectives said is a death threat against the commission's chairman, Johnston Kavuludi, the Daily Nation reported Thursday.
The box had been outside the building for 2 hours before police were alerted.
Nairobi police chief Benson Kibui said he believes the head and hands belong to a torso found in a farm in Ruai. He said the victim appears to be about 30 years old.
"Judging by the decapitation of the head and hands, we can be able to connect. We shall start by seeking the identity of the victim, then we find the killers and establish their motive," he said.
Kavuludi is the first chairman of the commission, which was created in October 2012 to enforce police reforms, the Daily Nation said.
Many of his orders have been ignored by senior police commanders, the newspaper said.
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