Bild reported on Thursday that five members of the cell in Wolfsburg, Lower Saxony, are currently fighting in Iraq, while two have already returned from trips to fight with Isis forces.
While one of them has been arrested, the other is "Ayoub B.", the target of the present investigation over plans for a "seditious violent attack".
A spokesman for the security services (Verfassungsschutz) told news channel n24 that the region around Wolfsburg, Lower Saxony's fifth-largest city around 75 km east of Hanover, is "a particular hotspot".
The security services are aware of around 40 people in total who have "left heading for Syria", he said.
Earlier this week, three Germans from Dinslaken were reported killed in fighting in Syria, while one of the city's Salafists was arrested at the weekend.