Spiral: The Bonds of Reasoning (2000)

Blurb:
World-class detective Kiyotaka Narumi’s last words prior to his sudden disappearance continue to haunt his younger brother, Ayumu. The cheeky 10th-grader becomes equally embroiled in the mystery of the doomed “Blade Children” when he is mistakenly named the prime suspect in a murder at his school. Led by Ayumu’s sister-in-law, Kiyotaka’s wife and fellow detective, Madoka, the investigation into the murder gives Ayumu a chance to clear his name. But in doing so, he not only uncovers ties to the Blade Children but also more questions than answers about who and what they are.

Source: www.anisearch.com/manga/2152