Garissa governor Ali Korane has been arrested.
He will be questioned on Tuesday over last week's shooting of former finance executive Idris Mkutar.
The governor was arrested by Flying Squad officers at his home and is locked up at Kilimani police station.
Two other suspects, believed to be hired assassins, have been arrested and are being held at Kilimani.
The executive was shot while at a shopping mall in Kileleshwa. He suffered serious injuries in the head.
He was left for dead outside a city hotel after which the DCI seized of the matter.