Investigators found DNA matching the 24-year-old men, identified only as Elwin and Yohan, in six sexual attacks between September and January in Marseille, France, the BBC reported Sunday.
Police say they are sure one of the men carried out the attacks -- perhaps both did -- but their DNA is too similar to differentiate between the two.
Police said they could conduct more sophisticated genetic tests to determine who is responsible, but it would cost upwards of $1.14 million.
"For a normal analysis, we would compare 400 base pairs (of nucleotides), which make up DNA," an expert told French newspaper La Provence.
For identical twins, "we would be looking at billions," he added.