A majority of the men who targeted women in an "organised" attack in Cologne, which saw many of them sexually assaulted on New Year's Eve, may never be caught, the city's police chief has said.
Juergen Mathies said that the CCTV footage for that night was not good enough to identify sex crimes.
The police chief said that authorities are relying on witnesses and victims to identify their attackers.
Mathies' predecessor, Wolfgang Albers, was suspended over the force's handling of the sexual assaults.
The attacks in the German city saw around 1,000 men target women.
German police say that more than 500 complaints were filed by women after the event, 40% of which were alleged sexual assaults.
Detectives have identified 75 suspects, with many of them coming from North Africa.
So far just 13 people have been arrested for theft, and one for sexual assault.
"The CCTV footage is not good enough to clearly identify sexual assaults.
"We can see some thefts but that's all. We are relying on witness accounts and on victims identifying their attackers," Mathies told the BBC.
On December 31, about 1,000 men, many of whom were of North African and Arab origin, congregated outside Cologne's main train station.
They dispersed into small groups and began threatening and attacking women.
Fireworks were also launched into the crowd.
The incident led to criticism of German Chancellor Angela Merkel's immigration policies.
Far-right groups staged a number of demonstrations in Cologne and other European cities against immigration following the attacks.
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