When one of them drowned, the two others tried to rescue him but met with the same fate, witnesses said
The dead have been identified as Moslema Akhter Shikha, 32, wife of log trader Harun-ur-Rashid, and their five-year-old son.
Both bodies were found with slash wounds on their necks from their home in Madarganj's Chargopalpur village on Wednesday, said Madarganj Police OC Rafiqul Islam.
Moslema’s husband Harun has been detained for interrogation over the deaths.
Police could not immediately provide any details about the possible assailants.
The bodies have been sent to Jamalpur General Hospital's morgue for autopsy.