A 47-year-old man accused of stabbing to death a former top health ministry bureaucrat and his wife and injuring the wife of another former official told a court on Thursday that he acknowledges the allegations in principle but pleaded not guilty to murder.

"I killed not people but demons with evil hearts," Takeshi Koizumi said at the Saitama District Court in the first hearing of his trial in a case that was initially thought to be an act of vengeance linked to the ministry's sloppy pension record management.

Koizumi said he "acknowledges in large part" the indictment, although it "contains elements different" from what he may have done.

Koizumi is charged with murdering former Vice Minister of Health and Welfare Takehiko Yamaguchi, 66, and his wife Michiko, 61, at their home in the city of Saitama on Nov. 17 last year, and also of attempting to kill Yasuko Yoshihara, 73, the wife of another former health and welfare minister, at their home in Tokyo the next day, although his defense council argued that he restrained himself "of his own accord."

At the opening of the trial, the prosecution said Koizumi had begun to hold a grudge against the health ministry when he was a junior high school student, believing that his missing pet dog had been killed by a healthcare center.

He then plotted to kill people who have had experience of serving as vice health minister when he turned 50, and wanted to get the death penalty, the prosecution said.

The hearings will focus on whether Koizumi will win leniency given that he turned himself in to the Tokyo police after the killings. The case is not being tried under the lay judge system, introduced in May, as he was indicted in March.

The verdict will be handed down on March 30, following closing arguments by prosecutors on Jan. 13 and by defense lawyers Feb. 10, according to the court schedule.