Investigators describe the unidentified couple, both in their 30s, as “persons of interest” in the case.

Their former neighbours in Samut Prakan’s Bang Saothong district told police they moved from their rented home soon after news broke on Sunday about a young girl’s body being found in Samrong Canal. The girl had died from asphyxiation.

The neighbours said the couple left without the woman’s 10-year-old daughter from a previous marriage, whom they claimed had gone to stay with relatives.

Bang Saothong police tracked the couple to Saraburi’s Phra Buddhabhat district, reporting the development to Provincial Police Region 1 chief Pol Lt General Charnthep Sesavech on Wednesday.

Interest focused on the 35-year-old stepfather, a delivery man, and 31-year-old mother, a factory cleaner, as police interviewed residents within a 10-kilometre radius of the location where the victim’s body was found.

Samran Jaipook, 68, who owns a grocery shop 200 metres from where rope was found of the same type used to bind the girl’s body, said the couple, who rented her house, had taken care of the 10-year-old girl since Songkran. Samran said the stepfather was hot-tempered and was often seen scolding the girl. Samran grew suspicious when police showed her photos of the murder victim’s body, bruised in the same area where her stepfather had allegedly struck her previously. The shopkeeper said she and other neighbours also recognised the victim’s clothing as belonging to the daughter.

On Tuesday morning police and soldiers raided a Bang Saothong shoe factory and workers’ dormitory and rounded up 22 migrant workers for DNA testing. No charges have been announced. People on the premises reportedly used rice from a Roi Et mill whose brand name appeared on a sack containing rocks and concrete blocks that had weighted the victim’s body when it was dropped in the canal.