The 15-year statute of limitations expired at midnight Monday for the 1991 murder of the academic who translated British author Salman Rushdie's disputed novel "The Satanic Verses" into Japanese.
With the expiration of the statute, prosecutors technically will no longer be able to indict the killer of Hitoshi Igarashi, but the police said they will continue to investigate the case.
They suspect it was a religiously motivated incident of international terrorism.
The statute of limitations will be suspended if the culprit is found to have been out of Japan for any period during the last 15 years.
Igarashi, 44, a researcher of Islamic studies and an assistant professor at the University of Tsukuba, was found dead July 12, 1991, with multiple stab wounds to his neck and face in a hallway of a campus building.
An autopsy found he had died the previous night at the earliest.
Igarashi translated the novel in 1990. It was originally published in 1989.
The book drew much criticism from Muslim leaders and followers worldwide as insulting their faith. Iran's Islamic revolutionary leader, the late Ayatollah Ruholla Khomeini, issued a "death sentence" to the writer over the novel.
Some 34,000 police investigators have been mobilized for the case over the years, but few clues were ever found.