The suspect confessed to planning to assassinate Mr. Abe a year ago.

Japan's NHK news station quoted investigative sources as saying that suspect Tetsuya Yamagami, 41, told police that he intended to use explosives to assassinate former Prime Minister Shinzo Abe in a plan that had been drawn up since last year.

Yamagami hatched the plan because he harbored a grudge against a religious organization he believed former Prime Minister Abe had close ties to. Japanese police have not revealed the name of the religious organization.

The suspect then changed his plan, using a gun to assassinate Mr. Abe instead of explosives, because his only target was Mr. Abe.

The suspect ordered parts and gunpowder online to make a double-barreled gun made from steel pipes, detonated with an electric trigger, and used shotgun-style bullets. He inserted a plastic shell containing six marbles into the steel tube filled with gunpowder, which could fire all of the bullets in one shot.

Police seized at least five other homemade guns from Yamagami's home. They also discovered that Yamagami may have test-fired the guns in deserted areas to test their lethality.

Suspect Tetsuya Yamagami at a police station in Nara city on July 10. Photo:Kyodo News.

The former Japanese prime minister was assassinated on July 8 while giving a speech in Nara city, western Japan. The suspect, Yamagami, approached him from behind and fired two shots from a distance of 5 meters, causing the former prime minister to die from blood loss.

Japanese law enforcement officials say the police team protecting Mr Abe did not follow VIP protection procedures and did not take action to shield the former prime minister when he was shot.

Yamagami said he hated the religious group he suspected Mr Abe had links to after his mother "made a huge donation" to them, leaving her in financial ruin. Police said they were investigating whether the former prime minister had links to the group as the suspect had claimed.