Police investigating the brutal murder of Mikio Miyazawa, his wife and two children in Setagaya Ward, Tokyo, now believe that one person was responsible for the killings, based on blood samples taken from the site of the crime.

Investigators at Seijo police station -- who had earlier believed that more than one person committed the crime -- say they have found traces of only one blood type that did not belong to the four victims in the house.

Miyazawa, a 44-year-old business consultant, his wife Yasuko, 41, and their 8-year-old daughter Niina were stabbed to death last weekend, while their 6-year-old son Rei was strangled.

In addition, police say they have found only one set of footprints -- matching a pair of men's sports shoes -- at the scene of the crime.

The killer is believed to have cut his hand with the weapon that he used to stab three of the victims, leaving traces of A-type blood in the second-floor bedroom and in the attic.

Police say they found only one blood type after an exhaustive study of the blood traces left at the scene.

Both Miyazawa and his wife appear to have resisted the intruder, struggling fiercely with him and, as a result, sustained several knife wounds.

Two chopping knives have been found in the house and police suspect that these were used, together with one other sharp-edged blade, in the killings.