The deceased were identified as Farida Begum, 55, wife of Ebadat Hossain, and their daughter Shahnaj Parvin, 25, a divorcee, of Bajruk Atitha village in Sadar upazila.
Police detained three people -- Jahedul Islam, 55, son of late Khaibar Rahman of Donil village in Badalgachhi upazila, Mamtaj Hossain, 50, and his wife Latifa Bibi, 45, Kanchan village in Manda -- for interrogation over the incident, said Hafizul Islam, senior assistant superintendent of police (Manda circle).
Mozaffar Hossain, officer-in-charge of Manda Police Station, said the victims along with Jahedul set out for Kusumba Mosque in Manda in the morning for offering special prayers there.
On the way, they went to the house of Mamtaj Hossain in Kanchan Bazar area, he said.
However, Farida and Shahnaj fell unconscious as soon as they came out of the house. They were taken to Manda Upazila Health Complex where doctors declared them dead.
Later, police recovered the bodies and sent those to Naogaon Sadar Hospital morgue for autopsy, the OC added.
Hafizul Islam said the death is shrouded by mystery.
He also said they detained the three people for interrogation over the incident.