The Interior Ministry identified the 31 suspects as nine Algerians, eight Moroccans, four Syrians, five Iranians, an Iraqi, a Serb, an American and two Germans. Most of the crimes they were accused of involved theft and violence, said a ministry spokesman, Tobias Plate, but at least three acts were considered sexual assaults.

The Cologne police chief resigned under pressure on Friday after days in which the authorities struggled to explain why they had not been able to control the violence and why they had been slow to make public the scale and nature of the assaults.

“It is all still incomprehensible,” said Ulrich Karpen, a professor of constitutional and administrative law at the University of Hamburg. “And the effect is that, in the general public, people no longer feel safe.”

It was not clear exactly when the migrants who may have been involved arrived in Germany. But the disclosure added to worries over acculturating hundreds of thousands of people from conservative Muslim societies — many of them young men — who have little experience with open European mores, particularly regarding women.