The motive of the assailant who carried out a knife attack on Tuesday in Kawasaki that left two dead and a dozen others injured remains unclear even after police searched his home, investigative sources said Friday.

Ryuichi Iwasaki, 51, who took his own life at the scene, had left a notebook in the house where he lived with his aunt and uncle. But its contents gave no indication about why the attack occurred, the sources said.

The suspect wrote the Chinese character sei or shou — meaning righteousness or real thing — again and again in the notebook.