A knife with traces of blood was discovered Monday in a house in Kanie, Aichi Prefecture, where a 57-year-old mother and one son were fatally stabbed and another son was wounded in an apparent home invasion, police said.
The knife had been wiped off but still bore blood traces, the police said.
The knife is believed to have been used by an unidentified attacker to stab to death Kihoko Yamada and her 26-year-old son, Masaki, last week.
The other son, Isao, 25, was stabbed and tied up in the home, although his injuries were not fatal.
The surviving son told investigators that the man who assaulted him spoke in a strange intonation when he demanded money, sources said.
Police suspect the intruder may not have been Japanese, the sources said.