Police who are investigating a series of sexual assaults in the southern French city of Marseille have arrested identical twin brothers.
The 24-year-old unemployed delivery drivers, named locally as Elwin and Yohan, were placed under investigation on Friday.
Officers say they are sure that one of the two men carried out the attacks, but that they do not know which.
Standard DNA tests are unable to differentiate between their DNA.
CCTV footage
There have been six similar sexual attacks in Marseille on women aged between 22 and 76 between September 2012 and January 2013.
Police got a lead from CCTV footage on a bus, and as a result they arrested the twin brothers. A victim also identified one of them as her attacker, but could not tell them apart.
The problem is that the investigators do not know which of the two men is responsible for the assaults, or indeed whether it could be both of them, reports the BBC’s Hugh Schofield in Paris.
Traces of the DNA of the perpetrator – or perpetrators – have been found on the victims.
But this does not really help, our correspondent adds, because the DNA of identical twins is itself more-or-less identical.
Police have been told it would cost upwards of 1m euros (£850,000) to conduct an ultra-sophisticated genetic test that would be able to tell one set of the twins’ DNA from the other.
One expert told the French newspaper La Provence: “For a normal analysis, we would compare 400 base pairs [of nucleotides] which make up DNA.”
In the case of identical twins, he added, “We would be looking at billions.”