Germany's justice minister says authorities need to quickly determine whether a string of New Year's Eve sexual assaults and robberies in Cologne may be linked to similar offenses in other cities.

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

Heiko Maas told Bild newspaper Sunday that if a group came together to commit such offenses, "no one can tell me that this was not coordinated or prepared."

Hamburg saw similar attacks on a smaller scale, and police in other European nations reported cases of comparable trouble in public places.

Maas says "all connections must be carefully checked."