On 23 June 2008, a fishing boat with 20 crewmembers onboard sank in reportedly moderate sea-state conditions in the Kuroshio Extension region east of Japan. To determine the sea state at the time of the incident, we conducted a hindcast wave simulation, as realistically as possible, using an improved third-generation wave model driven by wind and current reanalysis products. Our results indicated that at the time of the accident, the wave steepness increased and the spectral peakedness narrowed, creating a sea state favorable for freak wave occurrence due to quasi-resonance. Detailed analyses of the spectral evolution revealed that nonlinear coupling of swell and windsea waves was the key to generating the narrow spectrum. Under the influence of rising wind speed, the swell system grew exponentially at the expense of the windsea energy, and the bimodal crossing sea state transformed into a freakish unimodal sea.