Asylum seekers were involved in incidents of robbery and sexual violence in Cologne on New Year's Eve, according to the German interior ministry.
Dozens of women said they were robbed or sexually assaulted by men, with many witnesses claiming the attackers were of Arab or North African descent.
The incidents sparked uproar and large-scale protests in the country, with anti-immigrant groups questioning whether it was right to allow record numbers of refugees and migrants into the country last year.
Ministry spokesman Tobias Plate told a news conference that 18 asylum seekers were among the 31 people questioned.
- nine Algerians
- eight Moroccans
- five Iranians
- four Syrians
- two Germans
- one Iraqi
- one Serb
- one American
Mr Plate said the vast majority of the criminal acts were linked to theft and bodily injury, with some sexual assaults also reported.