The arrests come at a delicate time in German politics. Efforts by Chancellor Angela Merkel to negotiate a power-sharing arrangement after her Christian Democratic Union political party lost seats in recent elections collapsed over the weekend, raising two possibilities: a government that does not command a majority in parliament for the first time since World War II, or new elections that could result in greater empowerment of far-right parties opposed to Merkel’s open-door refugee policies.

German federal police special forces raided eight apartments in four states — Kassel, Hannover, Essen, and Leipzig — to arrest the men, who investigators said ranged in age from 20 to 28. Prosecutors said all had been ISIS fighters in Syria before using false identities to seek German refugee status between December 2014 and September 2015.

The arrests came just days after German security services announced that they’d opened four times as many investigations involving what they called “radical Islamists” this year compared to 2016.

BRUSSELS — In a series of dawn raids across four German states, police on Tuesday arrested six Syrian men suspected of being part of an operational ISIS cell planning an attack inside Germany.

About a million people fled conflicts or economic instability across the Middle East, North Africa, and Central Asia during 2014 and 2015. At least 800,000 Syrians and Iraqis were offered political asylum by the German government.

Investigators released few details of what the Syrians are suspected of planning. A statement said they were “planning an attack with weapons or explosives on a public target in Germany.” Multiple German media outlets reported that officials have said unofficially that the Christmas market in the city of Essen was considered a potential target and that the plot had developed well into “the planning stage.”

German police and prosecutors refused to speak about the matter on background but a European terrorism investigator — speaking on the condition of anonymity to preserve his relationship with German authorities — said the arrests had the hallmarks of perhaps the most sophisticated suspected ISIS cell to have arrived in Europe posing as refugees since a group of ISIS fighters and local radicals conducted attacks in Paris on Nov. 13, 2015, that killed 130 people.

“These guys had been ISIS in Syria and didn’t stop when they got to Germany,” the official who had seen a general briefing on the arrests told BuzzFeed News. “They remained in contact with one another across Germany, and it appears were still controlled by a central ISIS commander. It was clear that they were still operational and not just some guys who used to be fighters before fleeing the region.”

The suspected plan to attack a Christmas market — large outdoor shopping areas popular in northern European cities during the holiday seasons — was a stark reminder of the attack last December when a Tunisian asylum-seeker who’d been ordered deported drove a rental truck through a popular market in Berlin, killing 12.

On Sunday, the German newspaper Welt am Sonntag quoted German federal prosecutors as saying they'd opened more than 800 terror investigations into radical Islamists in 2017, compared to 250 last year. There were only 80 such cases in 2013.

While not all of the investigations involve planning for attacks in Germany, prosecutors told the paper, security officials have opened hundreds of investigations into the background of many asylum-seekers in order to hunt down militants who escaped security services at home and are living in Germany under false names.

The arrests came just weeks after another Syrian asylum-seeker, said to be 19 years old and to have traveled to Germany in 2015, was arrested by special police units while in the process of building a homemade bomb for use on a German target.

The man is suspected of having "planned and already concretely prepared an Islamist-motivated attack in Germany using very powerful explosives," a spokeswoman for the prosecutor's office, Frauke Kohler, told reporters on Oct. 31, according to the AFP news agency.

In a statement, the office said the man had made a decision "no later than July 2017 to explode a bomb in Germany with the aim of killing and wounding as many people as possible."