Three Syrian men believed to have been sent to Germany by the Islamic State group appear to have used the same smugglers and received passports from the same workshop as some of those involved in the Paris attacks.
The three men were arrested in northern Germany on Tuesday.
Interior Minister Thomas de Maiziere said they had been under observation for months and there were no indications of any concrete plans for an attack.
Federal prosecutors said the three are accused of coming to Germany in mid-November, supported by IS, "in order either to carry out an assignment they had already received or to keep themselves ready for further instructions".
They are suspected of membership in a foreign terrorist organisation.
Their arrests followed raids at refugee homes in Schleswig-Holstein, Germany's northernmost state.
"This may have been a sleeper cell," Mr de Maiziere told reporters.
He said the suspects appear to have made it to Europe through the same channels as two men who blew themselves up outside France's national stadium in November in the Paris attacks claimed by IS.
"Everything points to the same smuggling organisation as with the Paris attackers having brought these three who were arrested to Germany," he said.
"And everything points to the travel documents having come from the same workshop."
Prosecutors identified the suspects only as Mahir Al-H, 17, Mohamed A, 26, and Ibrahim M, 18, in keeping with German privacy rules.
The three travelled to Germany via Turkey and Greece, the route used by most migrants to Europe last year.
Mahir Al-H joined IS in Raqqa, Syria, earlier last year and received weapons and explosives training, prosecutors said, before he and the other two suspects in October told an IS official responsible for "operations and attacks outside the IS area" that they would travel to Europe.
The three were provided with passports by IS and were given a "high four-figure sum" of cash in US dollars as well as phones with a pre-installed communication programme, prosecutors said in a statement.
The Federal Criminal Police Office said that six locations in two German states were searched on Tuesday.
It said that, as well as the arrests, "extensive material" was seized as evidence.
The office noted that the attacks in Paris and Brussels over the past year showed that IS has used the migrant flow to send people to Europe.
It said that it knows of more than 400 tips about people among the migrants who might have a terrorist background, but most turned out to be wrong. However, more than 60 investigations were opened.
In July, two attacks were carried out in Germany by asylum seekers who arrived over the past two years and claimed by IS.
Five people were wounded in an axe rampage on a train near Wuerzburg and 15 in a bombing outside a bar in Ansbach. Both of the attackers were killed.
Those attacks and two others unrelated to Islamic extremism in the same week-long period put the country on edge and stoked tensions over the arrival last year of hundreds of thousands of migrants.
Those tensions have helped the nationalist, anti-immigration Alternative for Germany party, which won over 20% of the vote in a state election in the north eastern region of Mecklenburg-Western Pomerania on September 4.
Tuesday's arrests come days ahead of another state election in Berlin. Mr de Maiziere said they "show that security authorities are vigilant and are acting with determination".