STOCKHOLM—The debate over migration in Sweden has intensified after a local newspaper accused police of turning a blind eye to alleged offenses by asylum seekers in recent years.

Like in Germany, where authorities are grappling with the aftershocks of alleged mass assaults by migrants in Cologne on New Year’s Eve, the review of police action could have deep repercussions for how Europeans perceive migrants: as an opportunity or as a threat.

Swedish Prime Minister Stefan Löfven ordered police to investigate and disclose how they handled cases of alleged sexual harassment involving migrants at public events in the past two summers.

The instruction follows a report by Swedish daily newspaper Dagens Nyheter claiming that police had gathered information about refugees, some from Afghanistan, harassing teenage girls on the fringes of a festival in Stockholm in 2014, but failed to issue a warning ahead of the 2015 edition.

More cases of refugees groping young girls were recorded during last summer’s festival, the newspaper said, but police withheld the information over concerns it might feed ethnic tensions.

A spokesman for the police, Varg Gyllander, said that about 10 reports of sexual harassment had been received after last year’s festival.

“We have failed in our communication,“ Mr. Gyllander said. ”We should absolutely have commented on this. I have no good explanation as to why we didn’t at this time.”

Mr. Lofven said the incident was a double betrayal of victims because suspects haven’t been brought to justice and the police haven’t reported what has happened.

“The police should absolutely not try to hide anything, we have a problem here and we need to get a grip on it,” he told Swedish television.

The Swedish government has made a U-turn on migration policy in recent weeks. Sweden was the first country to grant permanent residency to all refugees arriving from war-torn Syria, and last year it took in more refugees than any other European state on a per capita basis.

Since Jan. 4, however, Sweden has imposed systematic controls on its border with Denmark, saying it had to reduce the number of new arrivals or public order would be threatened.

The refugee crisis has fueled tensions in the normally placid country with a recent spike in arson assaults against the homes of asylum seekers. In October, a student and a teacher were killed by a sword-wielding Nazi sympathizer in what police have described as a racist attack. The attacker was shot dead by police.

The far-right Sweden Democrats party has soared to nearly 20% in the polls and now competes for the position as the country’s second-largest party on an anti-immigration platform. On Monday Sweden Democrat lawmaker Kent Ekeroth said he would request that police chiefs appear in front of Parliament’s Justice Committee.