Identical twin brothers have been arrested by police in Marseille investigating a series of sexual assaults in the past 15 months. But the investigators have no idea which of the two brothers may be guilty, as the victims are unable to tell them apart.

Footage from closed-circuit cameras is unhelpful. Even standard DNA tests would prove inconclusive – the DNA of genuine twins is indistinguishable, except through experimental techniques which are costly and may not be accepted by a court.

The two men, who live together, are unemployed van drivers aged 24. They have been named only as Elwin and Yohan. One is believed to have committed six sexual assaults in Marseille on women aged 22 to 76 between September 2012 and last month.

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Both were arrested last Friday. Investigators hope that one might confess to clear his twin; or that one might incriminate his twin to clear himself. So far, both deny all accusations.

In 2011, Orlando Nembhard, 19, was identified as the killer of a man outside a nightclub in Arizona. But he was released without charge when it emerged that his identical twin, Brandon, was also present. Both denied the murder and police dropped the case.