Ashikaga murder case

On May 12, 1990, four-year-old Mami Matsuda went missing from a pachinko parlor in the city of Ashikaga, Tochigi Prefecture, Japan. She was found dead the following day at the nearby Watarase River. The case became known as the Ashikaga murder case and was later identified as part of a broader pattern referred to as the North Kanto Serial Young Girl Kidnapping and Murder Case, which encompassed a series of killings and disappearances of young girls in the Ashikaga area and neighboring Ota City, Gunma Prefecture, spanning from 1979 to 2005.

In 1991, Toshikazu Sugaya was arrested and subsequently convicted of Mami Matsuda's murder. His conviction rested on DNA evidence produced through testing methods that were, at the time, considered advanced but were later determined to be imprecise. In 2007, journalist Kiyoshi Shimizu, who had been given latitude to investigate the case after previous award-winning reporting, uncovered problems with the DNA testing method used to convict Sugaya. In 2009, when Sugaya's DNA was retested against the original evidence, the results conclusively showed that he was not a match, establishing his innocence. Sugaya was released in May 2009, after having spent seventeen years in prison.

Following Sugaya's exoneration, the prosecutor's office stated that the statute of limitations on the crime had passed, meaning the actual perpetrator could no longer be prosecuted for the 1990 killing. However, the statute of limitations had not yet passed on the most recent case within the wider North Kanto series, and multiple government officials, including then-Prime Minister Naoto Kan, publicly urged police to pursue a resolution of that outstanding case.

Kiyoshi Shimizu received the Editors' Choice Magazine Journalism Award for his reporting exposing the miscarriage of justice in the Sugaya conviction. In 2010 and 2011, Shimizu reported that he had identified strong evidence, including DNA evidence, pointing to an alleged perpetrator, and provided this information to police. No arrest resulted. Police reasoning cited a mismatch between the alleged perpetrator's DNA and DNA evidence previously attributed to the culprit in the original Ashikaga case. Shimizu maintained that the original DNA testing methods used in the case were flawed and suggested that an arrest under his theory would require the prosecutor's office to acknowledge those flaws. He drew a parallel to the Iizuka case, in which the same disputed testing methods had been used and the convicted individual was executed in 2008 despite requests for new DNA testing and a retrial.

The case remains formally unsolved with respect to identifying and prosecuting the actual perpetrator of Mami Matsuda's death, following Sugaya's full exoneration.