BERLIN — The German authorities on Friday tied asylum seekers for the first time to the wave of violent assaults on women in Cologne on New Year’s Eve as debate intensified over whether the country had made a mistake in opening its doors last year to more than a million migrants.

The Interior Ministry said 18 of the 31 people identified so far as suspects in the violence in Cologne had applied for asylum in Germany. The disclosure further stoked fears about security and culture clashes between the newcomers, mostly from Muslim countries, and Germans who are confronting the costs of assimilating them.

It also turned up the pressure on Chancellor Angela Merkel, whose welcome last summer to refugees from Syria and other countries in turmoil helped encourage migrants, and whose political standing increasingly rests on how she handles the challenge of absorbing them.

Sensing the souring of the public mood, Ms. Merkel shifted tone this week from her usual credo about integrating migrants — “We can do it” — to stiff condemnation, and even contemplation of expulsions or deportation for newly arrived offenders. The sterner note was embraced by an increasing number of German politicians and officials.