There were 38 reports of rape and sexual assault filed after the We Are Sthlm festival, which uses the postal abbreviation for Stockholm, in 2014 and 2015, according to police, who released the information on Monday after the Dagens Nyheter (DN) newspaper suggested that there had been a cover-up.
The paper reported it had seen a memo from last summer, warning police ahead of the event that there was a known "problem with young men who rub themselves against young girls" at the festival.
Police would not say how many men had been linked to the alleged assaults, but DN reported that as many as 50 Afghan refugees who had come to Sweden without their parents were suspected to be involved.
On Monday night, police said that a teenage boy, 15, had been charged with assault and sexual assault against two 14-year-old girls at the event.
Swedish artist Zara Larsson performing at the We are Sthlm festival. Photo: Alexander Tillheden/We are Sthlm
The free We are Sthlm festival is put on every year for 12 to 17-year-olds and is held in various locations in the city centre, including Kungsträdgården, a large park close to Stockholm's Royal Palace. The programme in 2015 boasted a number of international dance companies and circuses alongside Swedish artists including Zara Larsson, who headlined the event.
Police said last summer that there had been "relatively few crimes and people taken into custody considering the number of participants" in the festival, however documents sent by police to DN and the AFP news agency showed allegations of a total of 17 sexual assaults and one rape during the 2014 music festival, and 19 sexual assaults and one rape in 2015.
"We should certainly have written and told people about this, no doubt. Why it did not happen I do not know," Varg Gyllander, a police press spokesperson, told DN.
He said that police would reinvestigate exactly what happened and why the general public was not fully informed at the time.
Sweden's National Police Commissioner Dan Eliasson also swiftly tweeted that he would look into the apparent media blackout.
"We must go to the bottom with this. We will hand it over to internal investigations in the first instance to see if any errors or crimes have been committed," he told reporters later in the afternoon.
Blev varse om DNs uppgifter ikväll. Lovar att saken ska utredas.- Dan Eliasson (@dan_eliasson) January 10, 2016
Sweden's National Police Commissioner Dan Eliasson giving a press conference on Monday. Photo: Johan Nilsson/TT
DN's report on the cover-up by police came amid allegations that a psychologist also tipped off the newspaper about groping claims at the festival last summer and that Dagens Nyheter chose not to follow up the story at the at time.
A DN journalist contacted by news site nyheteridag admitted on Monday that she had contacted the police after speaking to the psychologist but said the story was never published because she did not get a response. The source told the site that the reporter had additionally suggested that her editor was not interested in the story either because it may have been fabricated by supporters of the nationalist Sweden Democrat party or because it could whip up racist sentiment in the city.
Nyheteridag is an independent online news site. Its editor is a formerly active member of the anti-immigration Sweden Democrat party.
The news editor of DN, Caspar Opitz, told The Local on Monday that he was unable to give an interview because his team was "working intensively" on the story.
In a comment published in Swedish on DN, he said: "We took it very seriously, had contact with the source and tried to move forward with the tip, but failed to get it confirmed".
"Information that DN has participated in a cover-up is a lie: something as important as young women being subjected to systematic abuse is clearly news DN would report," read a later statement on the site, in English.
He suggested that police had failed to provide a full picture of what happened during the festival and said that the newspaper had decided to publish its story this week only after gaining access to more concrete information about the alleged attacks.
Sweden's Prime Minister Stefan Löfven was also critical of the police and their handling of the incident.
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"I feel a very strong anger that young women are not able to go to a music festival without being offended, sexually harassed and attacked," he told reporters.
"It is a major democratic problem for the whole of our country," he said of the police failure to release timely information of the incidents to the public.
It is usually rare for information about alleged criminals' identities or ethnic backgrounds to be revealed during police investigations in Sweden or reported by the Swedish media.
However, the controversy over the Swedish festival comes amid a global debate about when connections to race should be made publically by police, following 121 sexual assaults in the German city of Cologne on New Year's Eve. Police chiefs there were also accused of attempting to hide asylum seekers' involvement in the reported crimes.