Shinkoiwa

Shinkoiwa is the setting of the riverbank murder that turns a suspected fire-related death into a second strand of the investigation into Michiko Oshitani’s killing. The place matters not merely because it lies near the Kosuge apartment, but because a corpse found in a burned shack is eventually identified as Mutsuo Koshikawa, the apartment’s occupant, exposing a connection between two crimes whose relationship the detectives have yet to explain.

Setting and Initial Discovery

The Shinkoiwa case began late on the night of March 12, when a makeshift tarpaulin structure beside the Arakawa River caught fire. A man’s body was found inside. At first, the death was treated as an accident, but the official autopsy established that the man had not inhaled smoke and had marks indicating strangulation. Those findings changed the fire from an apparent cause of death into a circumstance surrounding a suspected homicide.

The victim was initially presumed to be a homeless man known in the neighborhood, but the police had not established his identity. The uncertainty about who had died became central to the case: the man’s apparent life on the riverbank and his assumed local identity would later prove insufficient to identify the body.

How the Location Connects the Cases

Detective Shuhei Matsumiya raised the Shinkoiwa murder while discussing Oshitani’s death. The Oshitanis’ account suggested that Oshitani had last gone to work on Friday, March 8, and was absent beginning Monday, March 11; Matsumiya therefore placed her likely death on March 9 or 10. Since both cases involved strangulation and occurred only days apart, he suspected a connection. His colleague Ishigaki’s skeptical reaction, and Detective Kobayashi’s warning against linking crimes on dates alone, show that the idea was initially a working hypothesis rather than an established conclusion.

Matsumiya’s case for comparing the scenes was geographical as well as chronological. Oshitani’s murder was near the Arakawa River, while the Shinkoiwa scene was directly on its bank; he estimated that the sites were roughly five kilometers apart. The shared river corridor made the proximity specific, but did not by itself prove that the same person or plan connected the killings.

The autopsy evidence also left a meaningful difference between the cases. Both deaths involved suffocation, but Kobayashi understood Oshitani’s killing to have probably involved manual rather than ligature strangulation. The method supplied a point of comparison, not an identity of technique; the detectives had to keep resemblance separate from proof.

From Suspected Victim to Identified Man

A DNA comparison initially failed to connect the burned body to the missing Mutsuo Koshikawa. The comparison used DNA from material associated with Koshikawa’s apartment, and the early result established only that the body was not the missing man. This negative result prolonged the apparent separation between the Shinkoiwa death and the Kosuge apartment investigation.

The later test reversed that conclusion. DNA from the man who called himself Tanaka produced an almost perfect match with DNA taken from a toothbrush, razor, and towel in Koshikawa’s apartment. Tanaka had been living in the burned shack, had no fixed address or registered domicile, and earned money collecting empty cans. He said he had been away looking for food when the structure burned and had left the area because he feared being blamed; he also said he knew nothing of the theft of his personal items. The DNA, rather than the name he used or the life he described, identified him as Koshikawa.

The identification established that the corpse found at Shinkoiwa was Mutsuo Koshikawa, who had occupied the Kosuge apartment. It also disclosed that Koshikawa had previously been known as Shunichi Watabe, a man who had been in a long-term relationship with Kyoichiro Kaga’s mother. The discovery substantially advanced the investigation, but did not yet reveal why Oshitani and Koshikawa had both been murdered. The police requested background information from the Miyagi Prefectural Police, and at that point no useful lead had emerged.

Investigative and Narrative Significance

Shinkoiwa first enters the inquiry as a plausible parallel case: close in date, close to the Arakawa, and involving a similar cause of death. Its importance grows through a sequence of changing identifications. The corpse is first treated as an unknown homeless victim, then tentatively compared with Koshikawa, and finally established as Koshikawa by DNA from everyday objects in the Kosuge apartment. Each step narrows the gap between what the investigators see and who the dead man was.

That discovery changes the investigation’s direction without immediately solving it. Once the crimes could no longer be treated as unrelated deaths, detectives concluded that Oshitani and Koshikawa must have had some connection, even though the nature of it remained unknown. Shinkoiwa therefore functions as the location where a seemingly separate riverbank crime becomes evidence in a broader, still-unresolved relationship between the victims.

The later reconstruction of events also gives the Tokyo locations a personal dimension. Kyoichiro Kaga reasons that Oshitani may have gone to the Meijiza Theater’s opening-night performance, having stayed at a Kayabacho business hotel and bought one of the limited same-day tickets. He suspects she saw someone she recognized there: Tadao Asai, father of Hiromi Asai and a man believed to be dead. This theory does not establish what occurred at Shinkoiwa, but it places the murder investigation within a chain of concealed identities and recognition. The riverbank site’s decisive contribution is more concrete: it is where Koshikawa’s assumed disappearance becomes a confirmed death, and where the investigation gains a second victim whose identity points toward Kaga’s family history.