The suspect in Japan's worst mass murder in postwar history tied up his co-workers before methodically cutting the throats of his victims in what appeared to be a frenzied mission of mercy, investigative sources said.

Tuesday's knife attack at the Tsukui Yamayuri En care home for people with disabilities in Sagamihara, Kanagawa Prefecture, left 19 dead and 26 injured.

Satoshi Uematsu, a 26-year-old former employee of the facility, allegedly sliced the residents' necks as they slept early Tuesday, stabbing some in the chest or slashing their throats, the sources said.