Locals were alarmed by their barking dog refusing to be calmed down around 10:15 pm on Friday. After finding the injured woman in their garden with two male figures they called the police.
The witnesses took care of the victim and their presence lead to the suspects fleeing on a nearby bikeway.
According to the police the five boys are the main suspects in the rape but each of their roles in the crime is yet to be determined.
After questioning they were handed over into the custody of their parents during the weekend. One of them, a 14-year-old, is arrested again this morning.
The city does not want to have the boys partaking in any school activities while the investigation is ongoing and has asked for them to be relieved of mandatory attendance.
Social workers however were unable state whether any of the suspects did attend school this week.
Four of the five boys live in Mülheim. The police are still investigating the residency of the last suspect and were not aware about which schools they attend.
Social services were already working with two families prior to the rape but were unwilling to share further information.
They were also aware of the girls name beforehand and were asked to establish contact with her family for support.
The woman is being treated at a hospital and will receive further police assistance. Police declined to comment further to protect the victim during the investigation.
One of the 14-year-old boys was already known to police for a prior crime but no further information was given.
The police further refused to comment on a report by the tabloid Bild on whether or not the perpetrators filmed the attack.
In Germany children under the age of 14 are not criminally liable and can therefore not be prosecuted.
Social services will now work with the two families of the 12-year-old boys and determine further steps up to taking their children away.
Following this heinous crime the Police Union has restated their demand to lower the age of criminal liability to prosecution of younger perpetrators.