The alleged cover-up prompted a strong statement from Prime Minister Stefan Lofven.
"It's a double betrayal of these young women," Lofven told the 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."
The style of the attacks seemed to mirror a raft of assaults carried out on New Year's Eve in Cologne. There, groups of men repeatedly broke out from a 1,000-man crowd gathered near the main train station in town to surround, harass and steal from women, according to Germany's Spiegel Online.
Women throughout Europe reported similar assaults that night.
Of the more than 600 criminal complaints filed in connection with the Cologne attacks, about 40 percent involved sexual assault claims. At least two rapes were reported, according to Reuters. Police are focusing their investigation largely on asylum seekers or illegal immigrants from North Africa, and the wave of attacks has inflamed the refugee debate in Germany.
The attacks in Sweden in 2014 played out in a similar way, according to Britain's Guardian newspaper.
During the 2014 festival, organisers picked up on rumours of a new phenomenon, said Roger Ticoalu, head of events at the Stockholm city administration.“It was a modus operandi that we had never seen before: large groups of young men who surround girls and molest them,” Ticoalu said. “In the cases where we were able to apprehend suspects, they were with a foreign background, newly arrived refugees aged 17-20, who had come to Sweden without their families.”He said festival organisers did not have enough facts at the time to say anything definitive, and it would have been “totally irresponsible on our side to make anything public”. After the festival the organisers launched a programme with police and NGOs to encourage girls and young women to report harassment and to identify culprits, he said.
A spokesman for Stockholm police expressed regret over not sharing details of the attacks before they were published by Dagens Nyheter.
"I actually do not know why it did not happen," Varg Gyllander, head of communications for Stockholm police, told Radio Sweden, according to the BBC.
Gyllander and other police officials also admitted that the controversy over welcoming refugees and immigrants may have played a role in the reluctance to publicize the attacks.
"These days, the level of discussion is very harsh, and it's very aggressive when it comes to discussing the matter of refugees and foreigners," he said. "I think that all of us are very careful how we express ourselves."
In the fall, arsonists attacked more than a dozen Swedish refugee centers, including a home for unaccompanied refugee children, according to Britain's Telegraph. Police in Cologne reported at least four apparent reprisal attacks on foreigners on Sunday.