The former fighter was revealed to have been a suspect due to the damage and the nature of what was done to the victim, however we suspect the police have had more detail that simply haven't been released.
As a fighter Omagari made his debut in 1993 and despite having a pretty unspectacular start to professional boxing, going 6-4-3 (5) in his first 13 bouts he did go on to make a name for himself, winning his final 12 bouts by T/KO before retiring in 2007 with a record of 18-4-3 (17). During that run Omagari scored notable wins over Kazuhiko Hidaka and Tadashi Yuba whilst claiming the Japanese Welterweight title and recording 2 defenses of the belt.
His retirement in 2007 came following an eye issue that that he wanted to recover from.
At the moment clear motive has been given for the attack, and in fairness he's not actually been convicted, though it does seem like the police had more evidence on Omagari than we've been made privy to.
(Image courtesy of boxmob.jp)