Police in France investigating a string of rapes believe one, or both, of a set of identical twins is responsible.
But a DNA test to identify which of the two is the attacker could cost more than 1 million euros (NZ$1.6m).
Both twins, 24-year-old unemployed delivery drivers identified only as Elwin and Yohan, have been arrested and are being held without bail, French television news channel France 24 reported.
Police officers in the southern city of Marseille said they were sure at least one of the two men carried out the attacks, but they were unsure which one, or if both were involved.
Since last September six similar rapes have been carried out in the city on women aged between 22 and 76. The victims' mobile phones were taken in each of the attacks, which took place in the corridors of buildings.
CCTV footage on a bus gave police a lead and a victim also identified one of the twins as her attacker, but could not tell them apart.
DNA traces of the attacker, or attackers, were found on the victims.
But an expert told the French newspaper La Provence that with identical twins billions of pairs of the nucleotides which make up DNA would have to be compared because the DNA was so similar. Normally only about 400 pairs had to be compared.
"It's a rather rare case for the alleged perpetrators to be identical twins," chief investigator Emmanuel Kiehl said.
Police admit that without far more extensive tests it will be difficult to establish which of the twins was possibly behind the attacks, or if both were.
The twins deny any involvement in the rapes.