Garissa Governor Ali Korane has been arrested over attack on former county Finance minister Idris Aden Mkutar in Kileleshwa, Nairobi, on August 20.
Mr Korane was arrested on Tuesday morning and taken to the Nairobi Area Police Station for questioning.
Police officers, who are said to have camped outside the governor’s house in Kileleshwa overnight, pounced on him when he came out at 5am.
The officers went for Mr Korane after three other suspects in remand linked him to the attack.
At 8.30am, a convoy of vehicles, led by a chase car belonging to the Kilimani Police Station mobile patrol unit, entered the station with the governor in the back seat.
He was quickly taken into the station for interrogation and for hours, he responded to questions from Nairobi County Criminal Investigations Officer Nicholas Kamwende.
There is no cell at Nairobi Area and so it is unclear where he will go afterwards.
Mr Mkutar was shot in the head by a three-man gang outside a hotel in the capital.
Police investigations revealed that the gang trailed the victim to his car and shot him at close range before escaping without stealing anything from him.
The victim had left prayers at a nearby mosque and was leaving for his house when the gangsters opened fire.
He is still fighting for his life at a Nairobi hospital.
The other suspects in custody are a man accused of shooting Mr Mkutar, a woman who is said to have hired the car used to trailing the CEC and and a soldier.