The Isis plot to attacks the capital of North Rhine-Westphalia was bigger than previously assumed, with ten terrorists supposed to murder people with bombs and guns, local media report.
According to Spiegel, a suspect in police custody has admitted to authorities that ten people were supposed to be involved in the plot.
On Thursday three men were arrested in Düsseldorf, after a fourth plot member had turned himself in to authorities in Paris in February.
Three of the men, all of them Syrian nationals are alleged to have arrived in Germany in 2015 along the Balkan route, the route taken by refugees fleeing the Syrian civil war.
The six additional members of the plot were supposed to join them at a later point, Saleh A. who is in custody in Paris told investigators.
According to his account, two of the group were supposed to blow themselves up in the Altstadt district, the other men were then to attack more civilians with guns and explosives.
Police moved on the three group members in Germany on Thursday after they received information that one of the group wanted to travel to southern Europe and they feared they could lose sight of him.
The suspects were identified as 27-year-old Hamza C., 25-year-old Mahood B., and Abd Arahman A. K., 31, who were arrested in the states of Brandenburg near Berlin, North Rhine-Westphalia and Baden-Wurttemberg.
Saleh A., 25, also Syrian, had been in custody in France since turning himself in in February, they said, and Germany had now requested his extradition.
Saleh A. had gone to a police station in northern Paris on February 1, telling officers that he had "a certain amount of information about a sleeper cell that was ready to strike in Germany," a French judicial source said. He was held for four days and interrogated by anti-terrorist police.
"His statement led to investigations with German authorities which ... led to the arrest of these three suspects," the source added.