Media reports of the case have sparked outrage across Brazil.
In another incident earlier this year, a 23-year-old woman was jailed for one month together with 70 men.
The teenager, whose identity was not disclosed, was arrested in Abaetetuba, 80 km from Para state's capital Belem, on October 21 on suspicion of robbery and put behind bars at a local police station until an anonymous caller tipped off the media.
"Nobody really knows what she was charged with," Cohen said.
"She was a suspect in a robbery but police were unable to tell us which robbery. There was no formal charge."
The girl's lawyers said that police at one point said they believed the suspect was not under-age.
But Ana Julia Carepa, the governor of Para, said: "Whether she was 15, 20, 50, 80 or 100 doesn't matter. A woman should not have been kept in a jail cell with men." She said she would hand out "exemplary punishment" in the case.
The Cedeca spokeswoman said the girl is distraught and very afraid, but said she's able to recognise the police officers who locked her up and the detainees who raped her.
"She's ready to talk," she said.