Paris: French authorities are stuck with a riddle with no answer. They don’t know how to prove which one of the identical twin males accused of raping several women, is the one who did it.

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.

The DNA of one of the brothers was found at several locations from September last year to January this year, the time period during which six women were raped. Standard DNA tests are unable to differentiate between their DNA.

The rape victims — aged between 22 and 76 — have all been able to offer police positive identification of the rapist.

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, reports the BBC’s Hugh Schofield in Paris.

The brothers’ DNA is so closely matched that only few laboratories in France have the ability to perform such a test, which is also extremely expensive.

Emmanuel Kiehl, head of the security division that is conducting the research explains: “A normal DNA test is usually limited to 400 base pairs [DNA building blocks]. But with identical twins, the DNA is so similar it takes billions of base pairs to be examined to identify the perpetrator.”

Identical twins have very minute differences that occurred during the embryonic phase, making DNA testing very tedious and expensive.

Both brothers have no prior criminal record.

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.”