One of four suspects sought in a series of police raids conducted in three locations across Germany on Thursday had links to an Islamist network in Belgium, police said.
A 26-year-old Algerian man - one of four Islamist suspects, two of whom were arrested during the raids - travelled at least once to the Brussels neighbourhood of Molenbeek, where several perpetrators in terrorist attacks in Paris had lived or stayed.
The coordinated raids were conducted early Thursday in homes and businesses in Berlin, as well as refugee registration centres in Hanover and the town of Attendorn in North Rhine-Westphalia.
The police action took place after several weeks of investigations prompted by a tip-off of a terrorist attack from Germany's domestic intelligence agency, the Verfassungsschutz.
The main suspect, a 35-year-old Algerian man, was apprehended in the Attendorn refugee registration centre, while a 49-year-old man was taken into custody in the German capital, police said.
Police encountered the remaining two suspects - a 31-year-old and the 26-year-old with links to the Islamist scene in Belgium - but did not arrest them. The reason for this was unclear.
Police confirmed the men had been found to have links to Islamic State and that the main suspect had attended a training camp in Syria. Algerian authorities have a warrant for the man's arrest for being a member of a terrorist organization.
Police have information that the suspects were planning to meet in Berlin to plan terrorist attacks. It is not known whether the plans were close to fruition.
Frank Henkel, Berlin's senator of the interior, said that the threat of a terrorist attack in Germany remained high. "We continue to have every reason to remain vigilant and careful," he said.
Security sources told dpa that investigators have not managed to link the suspects to a failed terrorist plot in Munich and a thwarted attack at a friendly football match in Hanover.
Police also detained a woman during the raids in Berlin. There had been a warrant for her arrest on unrelated charges.