Montreal, July 24 (DPA) Canadian authorities have charged an Afghan immigrant, his wife and their oldest son with four counts of first-degree murder in the deaths of three Montreal sisters and an older female relative.

Zainab Shafia, 19, Sahar Shafia, 17, and Geeti Shafia, 13, all sisters, were found dead with Rona Amir Mohammed, 50, in a submerged car June 30 in the historic Rideau Canal near Kingston, Ontario.

From the beginning, investigators were baffled how the car managed to negotiate several obstacles to end up in the canal tail first, without leaving any skid marks.

Thursday at Kingston Police headquarters, Inspector Brian Begbie said that the teenage girls' father, Mohammad Shafia, mother Tooba Mohammad Yehya and brother Hamid Mohammad Shafia, 18, have each been charged with four counts of first-degree murder and four counts of conspiracy to murder.

Kingston Police Chief Stephen Tanner said that investigators are looking into whether the deaths were an 'honour killing'.

'That will form a part of the ongoing investigation,' Tanner said.

'Whether that was a part of the motive within the family, based on one... or more of the girls' behaviour, is open to a little bit of speculation.'

He refused to comment on rumours that one of the girls was pregnant out of wedlock.

It emerged during a press conference that Rona Amir Mohammed was actually Shafia's first wife, not a cousin as she was initially described by Shafia and reported in the media.