A 21-year-old former Self-Defense Forces member arrested over a fatal police box attack in central Japan was carrying at least four knives, a sign the attack was premeditated, investigative sources said Wednesday.

On Tuesday afternoon, Keita Shimazu, a part-time worker, stabbed to death Kenichi Inaizumi, the 46-year-old head of the police box in Toyama city, and stole his handgun. Shimazu then shot and killed Shinichi Nakamura, 68, a security guard at a construction site nearby.

(Keita Shimazu in his junior high school's yearbook)
[Supplied photo]

He was arrested at the scene after being shot by another police officer. He remains in critical condition.

The police found one knife at the police box, and Shimazu was holding two knives at the time of the arrest, the sources said. The police also found another knife in his backpack.

Shimazu told his family Tuesday before the attack through an online messaging app that he struck his boss at work and planned to quit his job, the sources said.

People at Shimazu's workplace told the police the incident actually took place before he ran off.

The attack shocked many in Japan as the country has strict gun controls and gun crimes are not common.

Shimazu joined the Ground Self-Defense Force in March 2015 and was stationed at Camp Kanazawa in Ishikawa Prefecture in central Japan, from April that year. He left the GSDF in March 2017, according to an SDF source.

On Wednesday, the police searched Shimazu's home in Toyama Prefecture. Investigators have found that all five bullets loaded into the stolen gun had been fired.

(Police search Shimazu's home.)

Autopsies showed Inaizumi had dozens of stab wounds mostly around his stomach, while Nakamura had gunshot wounds to his head and his left shoulder, they said.

Video footage taken by a security camera at the police box showed a man believed to be Shimazu with a black backpack walking by. Witnesses said he knocked on the back door of the police box shortly after and Inaizumi responded, according to the sources.

Nakamura's wife issued a statement, saying, "With (her husband) suddenly caught up in the incident, I cannot accept the reality."

An official at the security firm Nakamura worked for said Nakamura had been working there since August 2013.

"I'm very sad. He was just doing his regular job and it's hard to believe something like that happened," the official said.

In the wake of the attack, the National Police Agency plans to redesign police officers' gun holders to prevent others from easily snatching it. The agency originally planned to redesign it by around 2020.

Among incidents in recent years in which police officers were attacked, a man took a gun from an officer and fired four shots at him in Yokosuka, Kanagawa Prefecture in January 2016, seriously injuring the officer.