After a fatal attack on a 37-year-old in March, the police searched several apartments and rooms in refugee accommodation in Cologne and Wuppertal in the morning.
The police announced on Thursday that 18 men between the ages of 17 and 60 were being investigated.
A total of six apartments and four refugee accommodations in Cologne and one accommodation in Wuppertal were searched.
Several hundred police officers were involved in the operation.
The police said the men had been taken into custody on suspicion of joint manslaughter.
According to the police, the evaluation of video recordings of the attack led the murder commission to the suspects.
Together with nine other men who have not yet been identified, they are said to have fatally injured a 37-year-old man in Cologne in early March with blows, kicks and stabs.
The reason for the attack was disputes between extended families, police said.