Police stormed 54 homes, mosques and businesses in the state of Hesse in the early hours of the morning in an operation targeting extremists planning “serious state-threatening violence”.
“According to evidence gathered so far, attack plans were still in an early phase and had not selected a specific target,” said a spokesperson for the Hesse state criminal investigation office.
Officials said the main suspect arrested was a 26-year-old Tunisian man who has been wanted since August 2015 as a recruiter and people smuggler for Isis.
The unnamed suspect has allegedly established a network of supporters “with the aim, among other things, of committing a terrorist attack in Germany”.
He was present in Germany from 2003 to 2013, then returned during the refugee crisis of 2015 posing as an asylum seeker.
He was arrested shortly afterwards over a previous conviction for bodily harm in 2008, for which he needed to serve 43 days in prison, and Tunisian authorities had also requested his extradition.
They suspect the jihadi of involvement in the attack on the Bardo Museum in Tunisia in March 2015, which left 20 tourists dead, and an assault by Isis insurgents on the border town of Ben Guerdane last year.
German authorities said the man was freed from prison in September, and because Tunisia had not submitted the required extradition documents, he had to be released from temporary custody in November after the maximum period allowed under German law.
Paperwork delays also prevented the deportation of the Isis supporter who carried out the Berlin Christmas market attack, with the revelation sparking anger at German and Tunisian authorities as Angela Merkel pledged reforms.
“From immediately after his release until his arrest on 1 February, the main suspect was watched around the clock,” a spokesperson for Frankfurt police said.
The man did not resist his arrest, which was delayed to avoid jeopardising the ongoing investigation into a wider terror network, he added.
He is one of 16 suspected Isis supporters aged between 16 and 46 targeted in the four-month investigation.
In a separate case, prosecutors in Berlin said that they arrested three people on Tuesday night who were suspected of planning to travel to Syria or Iraq to undergo explosives and weapons training with Isis.
All three were associated with the Fussilet mosque in Berlin, known as a gathering point for radicals, prosecutors said.
Anis Amri, the Berlin Christmas market attacker, visited the mosque shortly before he ploughed a lorry into crowds on 19 December, killing 12 people.
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