A police officer was on Wednesday night shot dead by a colleague at Kinangop Police Station in Nyandarua County.
Sergeant Solomon Wainaina was pronounced dead on arrival at Kinangop Hospital where he was taken after the attack.
A trader at Engineer Market Njeri Munene said she heard the sound of gunfire shortly after midnight.
"We thought the attackers were criminals only to learn later that it was a fight between two police officers," said the trader.
She said the two officers have worked in the area for several years and are well known to the locals.
At the police station, a police officer who witnessed the incident said the sergeant had earlier picked a quarrel with his junior Justus Mwangi over the night duty shift.
The suspect returned from the night duty, found his senior in his office and they quarrelled for a second time before he shot the sergeant.
Kinangop police boss Huka Mohamed said the suspect is detained in police cells and that investigations are underway.
He said the suspect will appear in court after investigations are completed.