About 10 knives of the type believed to have been used to kill three members of a family of four late last month were sold in the vicinity of the victims' home, investigative sources said Thursday.

The sources said two knives were left in the Setagaya Ward home where 44-year-old company employee Mikio Miyazawa, his wife and two children were killed on the night of Dec. 30. The fourth victim, the Miyazawas' son, was strangled.

Miyazawa's mother-in-law confirmed that one of the knives belonged to the family. The other, however, was not at the house around 10 days before the incident, the sources said.

Police believe this knife was either purchased by the Miyazawas before the end-of-year holidays or was taken to the scene by the culprit, the sources said.

Investigators have so far been able to determine that about 10 knives of the same type were sold in Setagaya and Suginami wards in December, they said. One of these was sold on Dec. 29, a day before the killings, they said.

Cops ineffective: NPA

National Police Agency chief Setsuo Tanaka said Thursday that he believes the nation's police forces are losing their ability to prevent crimes.

Tanaka made the remark in referring to a series of heinous crimes that have been committed since late last year.

"Looking at the circumstances of these incidents that have shocked society, I've come to feel that the police have lost their ability to prevent crimes," he told a news conference.

The NPA chief also said minor incidents must not be overlooked when trying to maintain public order.

"The police and other law enforcement authorities must work to create an atmosphere in which illegal acts are punished according to the rules," he added.