STOCKHOLM (Reuters) - Swedish police on Monday ordered an investigation into allegations that officers covered up accusations of sexual assault by mostly migrant youths at a music festival in Stockholm, a scandal that drew criticism from the prime minister.

The investigation comes after internal police reports published in Swedish media said around 50 suspects carried out sexual assaults against teenage girls at the “We are Sthlm” youth festival last August. Assaults also occurred in 2014 at the same festival, which attracts thousands of people.

“It’s a double betrayal of these young women,” Prime Minister Stefan Lofven told Expressen newspaper. “It has not been prosecuted and handled in the way we would wish. The second is that police did not inform or tell about these problems.”

Swedish media have drawn comparisons to attacks on women in Cologne and other German cities on New Year’s Eve that prompted more than 600 criminal complaints with police suspicion resting mostly on asylum seekers.

Sweden faces a backlash among many voters after a record 163,000 asylum seekers arrived last year, part of a wave of immigration that has seen the far-right Sweden Democrats become one of the country’s biggest political parties.

This year the centre-left government has tightened border controls with identity checks on the main border crossing with Denmark after concerns authorities were unable to cope with the influx of refugees.

Quoting internal police reports and police who were there, Dagens Nyheter newspaper painted a picture of sexual assaults at the festival, which had escalated in 2014 and more so in 2015, committed mostly by young Afghans.

National Police Commissioner Dan Eliasson holds a news conference after Swedish police ordered an investigation into allegations that officers covered up sexual assaults by mostly immigrant youths at a music festival in Stockholm, in Salen, Sweden, January 11, 2016. REUTERS/Henrik Montgomery/ TT News Agency

According to police documents seen by Reuters, in 2015, police received 20 complaints of sexual crimes - 14 of which concerned girls under the age of 15 - in the area during the festival. In 2014 there were 18 complaints.

There was no mention of ethnicity of perpetrators in the files, and a police spokesman could not immediately provide a figure of how many of the cases had been prosecuted.

Police at the time described the 2015 event as one with “relatively few crimes”.

However, some officials said the police force was concerned that assault reports could have boosted the far right.

“This is a sore point. Sometimes we dare not tell how it is because we think it plays into the hands of the Sweden Democrats,” Dagens Nyheter quoted Peter Agren, who was in charge of police at the event in 2014, as saying.

National police chief Dan Eliasson said on Monday political considerations should not be part of their activities.

“We have to get to the bottom of this,” Eliasson told a news conference, adding the investigation would look at whether police officers had committed any crimes. He declined to discuss the ethnicity of the suspects.

Sweden Democrats lawmaker Bjorn Soder told Expressen: “It is an unprecedented scandal.”

Home Affairs Minister Anders Ygeman welcomed the police’s decision to investigate.

“It’s of course completely unacceptable if the police are concealing information,” Ygeman told TT news agency.