German authorities on Thursday arrested three Syrian men suspected of planning an attack in Duesseldorf for the Islamic State group, prosecutors said.
They said the plot was thwarted by a fourth suspect, who went to French authorities with details earlier this year.
The three men were arrested in three different German states, federal prosecutors said in a statement.
The plan called for two suicide attackers to blow themselves up in central Duesseldorf and then for further assailants to kill as many people as possible using firearms and explosives, prosecutors said.
However, they said there were no indications that they had started with concrete preparations.
The three men arrested in Germany were identified as Hamza C., 27; Mahood B., 25; and Abd Arahman A. K., 31. The suspect in custody in France was identified as Saleh A., 25, also from Syria. Their full names weren't released in keeping with German privacy rules.
Prosecutors said that Saleh A. and Hamza C. joined IS in Syria in 2014 and got instructions from the group's leadership to carry out an attack in Duesseldorf.
The two traveled to Turkey in May 2014 with IS approval, they added. In March and July 2015, they traveled separately via Greece to Germany.
The pair convinced Mahood B. to join the plot at some point before January this year, prosecutors said.
Also in January, Saleh A. contacted Abd Arahman A. K., who had traveled to Germany in October 2014 with instructions from IS to take part in the attack. According to prosecutors, Abd Arahman A. K. - who previously made suicide vests and grenades for the extremist Nusra Front group in Syria - was supposed to make the suicide vests.
The plot was thwarted when Saleh A. went to prosecutors in Paris February 1 with details of the plan. He has been in custody in France since. Prosecutors said that they will seek his extradition.
Prosecutors stressed that the arrests are not related to soccer's European Championship, which kicks off in France next week.
Germany so far has been spared mass-casualty attacks by Islamic extremists of the type seen in France and Belgium over recent months.