LONDON — The police in Sweden, responding to accusations of a cover-up, said on Monday that they were investigating why the public had not been informed about sexual assaults by men reported to be migrants at a festival in Stockholm last summer.

In echoes of the scandal in Cologne, Germany, where the police are investigating scores of assaults, often involving asylum seekers, on New Year’s Eve, the Swedish newspaper Dagens Nyheter reported over the weekend that a gang of migrant youths had groped young girls at a festival in August.

The attacks in Cologne have intensified the scrutiny of the assaults in Sweden, and even as Chancellor Angela Merkel’s spokesman, Steffen Seibert, said there was no excuse for revenge attacks against immigrants, a group of Pakistanis and a Syrian were targeted in Cologne on Sunday, The Associated Press reported.

The Swedish newspaper said that the police had not mentioned the attacks until it published details about them, and it accused the authorities of failing to warn the public before the festival that similar attacks had occurred in 2014, prompting speculation that the authorities were trying to avoid an anti-immigrant backlash.