The suspicious deaths of three Montreal sisters and a caregiver found inexplicably in a car submerged in the Rideau Canal took a dramatic turn Thursday as police alleged the four had been murdered by the girls' parents and brother in a possible "honour killing."

Kingston police Chief Stephen Tanner said investigators were looking into whether the parents and their 18-year-old son were motivated to kill the girls aged 19, 17 and 13, in a deadly clash of cultures.

The fourth victim was 52-year-old Rona Amir Mohammad. Police for the first time said she was the first wife of the older accused in the case, Mohammad Shafia.

The parents - Shafia and his wife Tooba Mohammad Yahya - and their 18-year-old son Hamed Mohammad-Shafia are charged with first-degree murder and conspiracy to commit murder.

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