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On Tuesday morning, 500 German police officers carried out anti-terror raids, detaining six Syrian refugees suspected of fighting for the Islamic State (ISIS). Authorities say they were planning a terror attack six days before Christmas. Reports suggest that the target may have been a Christmas market.

Prosecutors in Frankfurt said that six Syrian men between the ages of 20 and 28 were taken into custody. Roughly 500 police officers took part in the raids of eight apartments in the German cities of Kassel, Hanover, Essen, and Leipzig.

According to prosecutors, the men are suspected of “planning an attack with weapons or explosives on a public target in Germany.” Authorities added that the men had not yet finished preparing the attack.

Four of the suspects entered Germany as asylum-seekers in 2014 and the other two arrived in 2015, the German newspaper Deutsche-Welle reported. Refugees tipped off investigators about the men, who were reportedly active ISIS fighters in Syria.

The German newspaper Welt cited findings from the attorney general of Frankfurt that the Syrian suspects are ISIS supporters. They were arrested on suspicion of membership in a terrorist organization and preparation of a terrorist attack.

Welt also reported that investigators have come across terrorist suspects who had immigrated to Germany under false pretenses, claiming to be civil war refugees.

According to The Daily Mail, prosecutors say the suspected terrorists were planning an attack using “weapons and explosives.” The target may have been a Christmas market in Essen, and the attack planned for December 19. These details have not yet been confirmed by authorities, however.

On the same date last year, Tunisian ISIS terrorist Anis Amri hijacked a truck and murdered 12 at the Breitscheidplatz Christmas market in Berlin, Germany’s capital.

In March, police shut down a shopping mall in Essen after terror threats. At the end of October, police also detained a Syrian man who was also suspected of preparing an explosives attack in Germany.

Update: Suspects released.

The six Syrian nationals suspected of being ISIS members were released without charge on Wednesday after initial investigations found no “sufficient evidence of a terror plot,” the Frankfurt prosecutor’s office said.

Authorities added that there was “no concrete evidence” the suspects belonged to ISIS, either.

Even so, staff at the prosecutor’s office told CNN there are indications that the men had contacts with ISIS. Investigations are ongoing.

A German intelligence officer said the arrest of the Syrians uncovered a possible terror plot on the Essen Christmas market, however.