The man, identified by the police as Yuta Aizawa, a 21-year-old university student in Sendai, Miyagi Prefecture, visited the Higashi-Sendai police box in the city, around 4 a.m., telling 33-year-old senior officer Hiroaki Seino he had come to hand over lost cash.
A few minutes later, a 47-year-old police sergeant who was in a separate room heard shouting and found Seino and the attacker drenched in blood.
The sergeant warned the man to drop his weapons and shot once, but as the man continued to approach him, he fired two more times, according to the police.
Both Seino and the man were later pronounced dead at a hospital.
Seino had stab wounds in several places on his body including his chest and abdomen.
Aizawa used a kitchen knife with a 20-centimeter blade to stab Seino. He also had scissors, a box cutter and a screw driver in his waist bag when he arrived at the police box. A toy machine gun was also left at the scene.