A severed human head has been sent to the boss of Kenya’s police commission, Johnston Kavuludi, who has been trying to reform the service.
A box containing the head and two human hands covered in blood also had the message: “Kavuludi, you are next”.
The police say they are interpreting it as a death threat.
The BBC’s Gabriel Gatehouse in Nairobi says the incident appears to show that, in some quarters at least, opposition to reforming Kenya’s police runs deep.
He says Kenya’s officers are often described as among the most corrupt in the region.
Mr Kavuludi was appointed last year to lead efforts to reform the police force.
This was identified as one of the key challenges following the post-election violence five years ago in which more than 1,000 people were killed.
According to the Daily Nation newspaper, the box was covered in yellow polythene and was deposited in a flower bed outside the National Police Service Commission offices in central Nairobi.
The police were alerted about two hours after the box was left on Thursday afternoon.
Nairobi police chief Benson Kibui linked the head to the previous discovery of a torso on a farm outside the city, the Daily Nation reports.