The alleged rape of a woman by a group of teenagers in the Ruhr city of Mülheim has shocked Germany and sparked a debate about the age of criminal responsibility.

Five boys – three 14-year-olds and two aged 12 – were detained by police on Friday night in relation to a “grave sexual crime”.

An 18-year-old woman was treated in hospital after being targeted with what a police spokesperson described as “extreme violence”. She has since been released.

On Monday, prosecutors in the Ruhr region announced that they had imprisoned one of the 14-year-old suspects pre-emptively, given the severity of the charges and two previous reports of sexual harassment.

All five boys have been suspended from school until the start of the summer holidays on 15 July.

On Wednesday morning, German media reported that police were investigating another sexual assault by minors in the same city. In the latest incident, a 15-year-old girl was allegedly surrounded by five boys aged between 11 and 17, and subsequently harassed and touched inappropriately on Monday evening.

The Mülheim assaults come on the back of two high-profile gang rape crimes in Germany. In January, a regional court in Wuppertal found eight teenagers guilty of raping a 13-year-old girl near a swimming pool in the town of Velbert, in North Rhine-Westphalia.

In Freiburg, near the border with Switzerland, 11 men between the ages of 18 and 30 are on trial over the rape of an 18-year-old woman last October.

Under German law, children under the age of 14 are not held as criminally responsible. Instead, liability lies with their legal guardians or the local youth welfare office.

Compared with countries such as England, Wales and Northern Ireland, where minors can be held criminally responsible from the age of 10, the age of criminal responsibility is relatively high in Germany.

But the Mülheim case has led to some calls for the age of criminal responsibility to be lowered to 12. “It would be advantageous if youth welfare offices were no longer left alone to deal with 12- and 13-year-olds, and if juvenile courts could help the children at an earlier stage”, said Rainer Wendt, the head of the German police union, in an article in the tabloid Bild.

The German Association of Judges has dismissed Wendt’s suggestion. “The equation ‘more punishment equals less criminality’ does not work with youths,” said the association’s head, Jens Gnisa.

“The problem does not lie with our criminal law, but the families and parents that the state holds responsible for raising and educating young people,” commented the broadsheet newspaper Frankfurter Allgemeine Zeitung. In the Mülheim case, some of the suspects’ families had reportedly rejected help from youth welfare officers.

In theory, youth welfare officers have the authority to intervene and even remove minors from their parents if they find that families cannot control their children alone.

Police statistics in Germany from the last three years show an overall drop of suspects under the age of 14 held in connection to crimes. However, the number of teenagers younger than 14 held in connection with “crimes against sexual self-determination” has risen over the same period, from 1,420 to 2,118.