Police in the German city of Cologne said Sunday that 516 criminal complaints had been filed in connection with a series of attacks during New Year's celebrations, including over 200 allegations of sexual offenses.

Authorities and witnesses say the attackers were among a group of about 1,000 people, mostly men, gathered at Cologne's central train station, some of whom broke off into small groups that groped and robbed women.

Police have said they are focusing on suspects of primarily North African origin, which has put pressure on Chancellor Angela Merkel's government and its open-door policy to asylum seekers. Nearly 1.1 million migrants arrived in Germany in 2015 alone.

Meanwhile, Germany's justice minister said Sunday authorities need to determine whether the attacks in Cologne were linked to reported assaults in other European cities. 

"If such a horde gathers in order to commit crimes, that appears in some form to be planned," Heiko Maas told the newspaper Bild. "Nobody can tell me that this was not coordinated or prepared."

Police in the northern city of Hamburg are also investigating sexual assaults and thefts in the St. Pauli district which occurred on a smaller scale than in Cologne. Authorities in Sweden and Finland are also investigating similar incidents in their countries.

"All connections must be carefully checked," Maas said. "There is a suspicion that a particular date was chosen with expected crowds. That would then be a new dimension."

Of an initial 31 suspects detained by police for questioning, 18 were asylum seekers but there were also two Germans, an American and others, and none of them were accused specifically of committing sexual assaults.

Police have released few details, but Bild on Sunday said one, a 22-year-old Tunisian, was registered at a refugee center in a neighboring state, while two Moroccans aged 18 and 23, were apparently in the country illegally, according to their attorney.

"Our clients are modern nomads," attorney Ingo Lindemann told the newspaper. "They're not war refugees but more like grown street children who move with the flow of refugees across Europe."

Cologne police wouldn't confirm the report on the three and Lindemann didn't immediately return a phone call or an email from the Associated Press. 

Also Monday, Swedish police faced allegations of a cover-up for failing to inform the public of widespread sexual assaults against teenage girls at a music festival last summer.

Police hadn't mentioned the August incidents at the "We are Sthlm" festival until newspaper Dagens Nyheter reported on them this weekend following a string of sexual assaults and robberies on New Year's Eve in Cologne, Germany.

Stockholm police spokesman Varg Gyllander confirmed to The Associated Press on Monday there was "a large number" of sexual assaults during the five-day festival and that scores of suspects were detained.

He said police should have reported on the incidents at the time "given the nature of the crime." He denied suggestions in the newspaper report that police kept quiet because the suspects were foreigners.

"We probably should have communicated this," Gyllander told the AP. "But we wouldn't have discussed ethnicity at all."

German police also announced that on Saturday night they arrested a 19-year-old Moroccan man on allegations he stole a 23-year-old woman's cellphone on New Year's, and had identified 19 other suspects by name. Police said the Moroccan, who they said has been involved in various crimes since 2013, isn't accused of a sexual offense but the investigation is ongoing.