The statute of limitations in the case expires in 5 years.
Citing insufficient evidence, prosecutors on Wednesday released four individuals held in connection with the 1995 shooting of the nation's former top police official-a development that must be hugely embarrassing to law enforcement officials.
Tokyo prosecutors said the case would not stand up if it went to court, and in the absence of further evidence they had to allow the suspects to go free.
Much of the case rested on statements given by Toshiyuki Kosugi, a former police officer and member of Aum Shinrikyo, the cult that carried out deadly sarin gas attacks in Tokyo and Nagano Prefecture. Kosugi initially denied he shot Takaji Kunimatsu, but he later changed his story and said he may have shot the National Policy Agency chief. The lack of physical evidence placing the suspect at the scene added to difficulties in getting indictments.
Kosugi was released along with Tetsuya Uemura, 49, and Mitsuo Sunaoshi, 36. All three, who were cult disciples, were arrested July 7 on suspicion of attempted murder for assisting the gunman.
Koichi Ishikawa, 35, another former cultist, was arrested in a separate explosives case but was suspected of involvement in the shooting outside Kunimatsu's residence. He, too, was released.
The four could still be indicted if further evidence turns up linking them to the case, sources said.
But given the fact that nine years have already passed, the possibility of that happening would seem unlikely.
Kunimatsu himself seemed unperturbed by Wednesday's developments.
"Speaking from the standpoint of the victim, the latest investigation I believe has clearly shown that Aum was involved in an organized manner,'' Kunimatsu said.
"There are times when things go right and others when they don't,'' Kunimatsu told reporters. "It is not clear who actually fired the shots. Without the gun used in the incident, it cannot be helped that things worked out the way they did today.''
Kunimatsu, who was struck by three bullets in the shooting, said he still experiences stiffness in his right leg, especially during cold weather.
The statute of limitations in the case expires in about five years.
While the investigation will continue to focus on Aum members, police now will have to make a more concerted effort to dig up solid evidence to prove their case rather than rely on individuals who keep changing their stories.
What rings loud and clear is that the police were wrong to believe they could build a case by simply questioning a group of people they thought might be connected with the case.
Of the four, only Kosugi had ever hinted at involvement in the shooting. The others either refused to talk to police or denied any wrongdoing.
A source close to the investigation said police went ahead with the arrests because they did not expect new evidence to turn up, no matter how long they continued investigating the case.
With the investigation approaching its 10th year, the police were well aware of rising public criticism about the lack of progress and waste of public resources.
There are still about 100 officers assigned to the case.