Inspector General Joseph Boinnet’s driver, Anthony Lemayan Lenkisol, died by suicide on Sunday while on sick leave.
Evans Leteipa Maasai, a man who identified himself as the 45-year-old's brother, reported that he died at Mwamba estate on the outskirts of Narok town.
Police are piecing together information to establish why Mr Lenkisol had his firearm while off duty.
Their report says the General Service Unit officer based at Mr Boinnet's office in Nairobi, shot himself about 50 metres from his house.
He pulled the trigger after placing his Jericho pistol underneath his chin.
The report adds that the body had two gunshot wounds, one under the chin (the bullet's entry point) and the other in the head (the exit wound).
Police are looking into why Mr Lenkisol was allowed to take his official firearm with him when he took the leave.
It is against police standing orders for an officer to have the official weapon while off duty.
On Friday, Narok County Police Commander Thomas Ngeiwa clarified that Mr Lenkisol was not the IG's personal driver.
Mr Ngeiwa said the driver's brother recorded a statement at 11.10pm that fateful day, under OB 16/9/12/2018.
"In the incident report, Mr Leteipa confirmed that his brother had committed suicide," he said, adding the burial will take place on December 17 at his Kisirir home.
The police boss said officers found the pistol, a spent cartridge and a magazine with 14 bullets at the scene.
He said their investigations will unearth the reasons for the incident.
Mr Lenkisol had appeared before the promotions board and had been awaiting confirmation of his elevation to the level of a corporal.
A source in his family said a road accident last year left him mentally unstable and that he had been on medication.
Mr Ngeiwa said it was not clear why the officer would be allowed to have a gun under these circumstances.
“Investigations are ongoing both in Narok and Nairobi," he said, adding updates will be given.
The body was taken to Narok County Referral Hospital morgue.
The suicide came about a month after an Administration Police office in Kilgoris shot his wife dead before stabbing himself using a knife following a suspected domestic row.
It was said that the officer had been on a drinking spree and that he shot the woman four times.
Neighbours responded after hearing their five-year-old daughter screaming.
At the Kenya School of Government in Mombasa, where he met coastal region commanders in November, Mr Boinnet said they were worried about the high number of such incidents.
Cases have also been reported of police officers turning their guns on civilians, and the civilians retaliating.
“We want to support officers who require help because of the tremendous stresses that police work exerts on us. Majority of police officers work very well but a few, occasionally, break down,” Mr Boinnet said.
“We have created a department of counselling and are in the process of decentralising the service to regions. With enough resources, we will go further into the counties."
The National Police Service Commission established a Counselling and Rehabilitation Centre in October to address psychosocial or stress-related problems affecting officers