The parents of a 14-year-old boy who was killed in what a family court deemed to have been a one-on-one fight with another boy are to file a damages suit next week against the state and Ibaraki Prefecture, claiming the investigation was unfair, it was learned Friday.

They filed a 100 million yen damages suit Thursday with the Tokyo District Court against the other boy.

The lawsuits are aimed at shedding more light on the case. They are also likely to provide another topic for debate on the opaque juvenile crime investigation process in Japan as they come at a time when revisions to the Juvenile Law are being considered.

Kimio and Kazue Okazaki, from Ushiku in the prefecture, will file the suit against the government with the Mito District Court. They are charging Ibaraki Prefectural Police and the Mito District Public Prosecutor's Office with compiling investigation reports on the 1998 death of their son, Satoshi, based only on statements made by the other boy.

Officials at the Ibaraki Prefectural Police said Friday that they had found the boy's confession trustworthy since it was consistent throughout their investigations, adding that he was the only person who could have known what happened.

The media initially reported the case as a one-on-one fight based on police information released several hours after the incident.

Based on the investigation, the inquiry at the Mito Family Court's Tsuchiura branch determined last Aug. 25 that Satoshi had started the fight with one of his schoolmates in a forest near their junior high school on Oct. 8, 1998.

After the schoolmate hit Satoshi's head and back, he fell to the ground, slipped into a coma and died shortly afterward, according to the inquiry.

The family court placed the other boy on probation.

The plaintiffs' lawyers said the investigation may have tried to water down the criminal nature of the case from the very outset, since the other boy's father and brother were police officers at the time.

Statements found in the arrest warrant for the boy show that police had already formulated a scenario in which a "one-on-one fight with bare hands" was provoked by the victim, although not enough time had passed in which to collect the facts, according to the suit.

The plaintiffs also claimed that investigative authorities unfairly neglected some facts in their reports, including one witness who stated he saw several boys entering the forest and heard them shouting as if they were "hunting a dog."

The family court, noting that Satoshi had a minor heart problem, concluded that stress cardiomyopathy, induced by moments of excitation, caused Satoshi's death.

However, Masahiko Ueno, former director of Tokyo Medical Expert's Office, said in his opinion attached to the damages suit that he believes the victim suffered severe injuries to his abdomen, as if by kicking or kneeing, which caused neural shock.

He cited the victim's underwear -- soiled with bloody urine -- as key evidence to support his view. He added that medical experts appointed by investigative authorities and the court might not have seen the bloody underwear.

"Police concealed evidence in an attempt to misdirect experts' opinions," the plaintiffs charged in the suit.

Ibaraki Prefectural Police officials defended their actions, saying they have carried out all the investigations possible and adding it is "both impossible and unnecessary" for them to have conducted the probe with a view to favor a member of a police officer's family.

The parents began to question the investigation based on their strong belief that their son would never hit anyone back. After the parents and lawyers started their own probe of the case, the Diet also began looking into the issue early last year.

The moves prompted police to launch a second investigation in April. The results, however, were the same as those of the initial probe.

The Okazakis and their lawyers are organizing a gathering Monday in which victims of juvenile crimes as well as Diet members will discuss what is needed in probes of such cases.

Hiroaki Soejima, one of the plaintiffs' lawyers, said that fact-finding in juvenile cases largely depends on investigative authorities' discretion and lacks precise rules regarding such issues as what evidence is admissible, which exist in adult criminal trials.

There is a premise under the current system that investigative authorities function fairly, he added.

However, "as evidenced by the recent series of police scandals, it would be of no surprise to learn that the investigation of the (Okazaki) case may have been carried out in a way that diminished its (gravity)," lawyers said in the written complaint.

Satoshi, then captain of Ushiku No. 1 Junior High School's soccer club, was known as a talented player who won the Ushiku City Sports Award in eighth grade. He was also recognized by his school for his "sense of justice."

"How my son died . . . (is what) we really want to know," Satoshi's father, Kimio, said when explaining the reasons for the suit. "There is no other way to find out."