Police are leaning toward the theory that robbery was the probable motive behind the slaying of a Fukuoka family in June, investigative sources said Thursday.

The Fukuoka Prefectural Police believe the three suspects in the case, two of whom are being held by Chinese authorities, initially planned only to steal money or other valuables.

Discovered June 20 in Hakata Bay were the bodies of Shinjiro Matsumoto, a 41-year-old clothing merchant from the city of Fukuoka; his 40-year-old wife, Chika; their 11-year-old son, Kai; and their 8-year-old daughter, Hina.

The bodies were handcuffed and weighed down by dumbbells.