WIESBADEN, Germany/BERLIN (Reuters) - A 36-year-old Tunisian asylum-seeker arrested on Wednesday on suspicion of planning an Islamist attack in Germany is also wanted in his homeland over a deadly 2015 assault on a Tunis museum favored by Western tourists, German officials said.

German special police forces stand guard in front of Frankfurt's Bilal mosque during early morning raids in the federal state of Hesse and its capital Frankfurt, Germany, February 1, 2017. REUTERS/Kai Pfaffenbach

The Tunisian is suspected of recruiting for Islamic State in Germany since August 2015 and building up a network of supporters with the aim of carrying out a terrorist attack, the Frankfurt prosecutor’s office said in a statement.

He had lived in Germany for a decade until 2013, before re-entering the country to seek asylum in August 2015, it said, five months after Islamist militants stormed the Bardo Museum and killed 21 foreign tourists.

“The main suspect is a...Tunisian citizen strongly suspected of working as a recruiter for the foreign terrorist organization that calls itself ‘Islamic State’...with the aim of carrying out a terrorist attack in Germany,” the statement said.

The German newspaper Die Welt identified the Tunisian as Haikel S. and said he had been known to German security agencies as an adherent of the ultra-conservative Salafist branch of Sunni Islam for the past decade.

Die Welt quoted investigators as saying he had been in contact with an Islamic State cell responsible for “external operations” and had planned attacks in Europe. Reuters could not immediately independently verify the report.

Germany is on edge over Islamist violence after a spate of attacks last year, and how a Tunisian who had been on spy agencies’ radar could re-enter the country undetected may heighten public doubts about Chancellor Angela Merkel’s open-door immigration policy ahead of September’s federal elections.

Merkel, seeking a fourth term in office, has come under heavy fire from right-wing opponents for allowing more than a million asylum-seekers into the country over the past two years.

Interior Minister Thomas de Maiziere declined to say when authorities became aware that the Tunisian suspect was back in Germany. He also called for increased surveillance of Salafist mosques in Germany involved in radicalising Muslims.

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“States must put mosques known to be involved in radicalisation under observation and close them if necessary,” he told the Nordwest-Zeitung newspaper.

The Tunisian was arrested in a security sweep in which more than 1,100 German police raided 56 homes, businesses and mosques in Frankfurt and other towns in the western state of Hesse.

Peter Beuth, interior minister of Hesse, said there had been no immediate danger and the arrests were pre-emptive. The raids followed four months of investigations and destroyed “an extensive Salafist network”, he said.

CONNECTION TO TUNISIA ATTACKS

The museum attack was the first major militant strike in Tunisia since the country’s pro-democracy uprising in 2011. In June 2015, an Islamist gunmen shot dead 39 people, mostly British holidaymakers, in the Tunisian seaside town of Sousse.

Tunisian investigators suspect the Tunisian asylum seeker was involved in “planning and carrying out” the Bardo Museum assault and had issued a warrant for his arrest in June 2016, the prosecutors’ statement said. He was also suspected of a role in an attack on a Tunisian border town in 2016, it said.

German authorities had no immediate comment on a report by German news agency dpa that Tunisia was seeking his extradition.

The Tunisian suspect was detained in Germany in August 2016 in connection with a 2008 conviction for bodily harm, but his extradition to Tunisia fell through when Tunisian authorities failed to provide the necessary paperwork. He had been under surveillance since his release in November, the prosecutor said.

Die Welt reported that he was arrested on suspicion of being an Islamist militant but was released due to insufficient evidence that he was a member of Islamic State.

Authorities are investigating 15 other suspects aged between 16 and 46, of whom 13 are suspected of preparing “a serious act of violent subversion,” the Frankfurt prosecutor general said.

Among those being investigated are a 17-year-old German-Iraqi man and a 16-year-old German-Afghan on suspicion of taking lessons from an Islamist militant group in how to use firearms and explosive devices.

In an unrelated case, a 31-year-old German man was arrested near Nuremberg on suspicion of belonging to “a radical Islamist terrorist militia” in Syria in 2013 and 2014, according to the state prosecutor’s office in Duesseldorf.

German has been on high alert since a failed Tunisian asylum-seeker ploughed his truck into a Berlin Christmas market, in December, killing 12 people. He was later shot dead by police in northern Italy.

On Wednesday, the German cabinet approved allowing federal police to fit suspected militants with electronic tags, a step that will need final consent from the Bundestag (lower house of parliament).