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Martin Meissner, Associated Press
A German flag hangs in a window at a refugee shelter in Recklinghausen, Germany, Sunday, Jan. 10, 2016. German police on Saturday raided an apartment at the shelter for asylum-seekers in the western city of Recklinghausen that they say had been occupied by a man shot and killed as he stormed a Paris police station. North Rhine-Westphalia state police said in a statement late Saturday that they were acting on "concrete evidence" they had obtained from French security authorities.

BERLIN — Authorities need to quickly determine whether a string of New Year's Eve sexual assaults and robberies in Cologne blamed largely on foreigners may be linked to similar offenses in other cities, Germany's justice minister said in comments published Sunday.

Authorities and witnesses say the attackers were among 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.

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

The attacks are still being investigated, but police have said their focus is 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.

She announced a proposal Saturday that would make it easier to deport migrants who commit crimes, which still needs parliamentary approval.

In a development that seemed likely to provide more grist for opponents of Merkel's policies, police in the western state of North Rhine-Westphalia on Saturday raided an apartment at a shelter for asylum seekers in Recklinghausen that they said had been occupied by an extremist shot and killed last week as he stormed a Paris police station.

French Interior Minister Bernard Cazeneuve said in remarks Sunday that the man was also believed to have lived in Luxembourg and Switzerland.

Separately, police in Hamburg are also investigating sexual assaults and thefts in the St. Pauli district similar to those in Cologne, which occurred on a smaller scale in the northern city on New Year's Eve. 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."

Cologne police are investigating 379 criminal complaints filed with them, about 40 percent of which involve allegations of sexual offenses.

So far, of 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 would not confirm the report on the three and Lindemann didn't immediately return a phone call or an e-mail.