Prosecutors decided Friday not to indict four men linked to the AUM Shinrikyo cult over the 1995 shooting of the National Police Agency chief due to a lack of evidence.
The four, including former police officer Toshiyuki Kosugi, 39, were arrested in early July but were later released.
The Tokyo District Public Prosecutors Office said it decided not to indict them because it had doubts about the credibility of confessions by Kosugi, who changed his statements after arrest.
"We have continued investigations to catch the suspects and uncover the truth. It is regrettable that investigators could not collect enough evidence for the prosecutors to request a trial," said Seishi Suei, public security head at the Metropolitan Police Department.
The prosecutors and police had continued questioning the four on a voluntary basis after their release, but apparently did not find any substantiating evidence.
The three others are Tetsuya Uemura, 49, Mitsuo Sunaoshi, 37, and Koichi Ishikawa, 35. Ishikawa was arrested over an explosion at a religious scholar's house.
The NPA commissioner general at the time, Takaji Kunimatsu, was shot and severely wounded in front of his home in Tokyo's Arakawa Ward on March 30, 1995, eight days after the police launched raids on AUM following the cult's fatal sarin gas attack on the Tokyo subway.
The police arrested the four July 7 after gunpowder found on a coat belonging to Kosugi matched that from a bullet in the shooting. Kosugi originally told investigators he had lent his coat to a man believed to be the shooter.
However, Kosugi changed his statement after being arrested to say he was the one who shot Kunimatsu, stirring doubts about his previous remarks.
The prosecutors released the four July 28 when their legal detention period expired.
In 1996, Kosugi confessed to being the shooter, but prosecutors doubted his credibility and deferred indictment.