BERLIN (Reuters) - A German judge authorised on Thursday the arrest of a 26-year-old Iraqi man detained after an attack on a bus carrying players of a soccer team, and prosecutors said they believed he was a member of Islamic State.

The Federal Prosecutor’s Office said the man, identified as Abdul Beset A., had joined Islamic State in Iraq by the end of 2014 at the latest and had led a 10-man command unit there. The jihadist group controls parts of Iraq and Syria.

“The task of his unit was to prepare abductions, kidnappings, extortions and killings,” the prosecutor’s office said in a statement.

The man travelled to Turkey in March 2015 and from there on to Germany in early 2016.

“Also from Germany, the accused continued to maintain contacts with members of ‘IS’,” it added, citing the grounds for the arrest.

Earlier, the prosecutor’s office there was no evidence that the man had taken part in the attack in the western city of Dortmund.

Three explosions went off as the Borussia Dortmund bus travelled to the club’s stadium for their Champions League match against AS Monaco on Tuesday, injuring Spanish defender Marc Bartra and delaying the fixture by a day.

Security is an issue in campaigning for Germany’s federal election on Sept. 24, a tight battle in which Chancellor Angela Merkel is seeking a fourth term. In December, a Tunisian man killed 12 people when he ploughed a truck through a Berlin Christmas market.

Last year, Merkel lost support to the anti-immigration Alternative for Germany (AfD) party after opening the country’s doors to more than a million migrants. The AfD has since seen its ratings fall while the chancellor has regained some poise.

MILITARY DETONATORS

Bild newspaper said on Thursday that military detonators had been used to set off the three explosive devices, probably remotely using a mobile phone.

Three identical letters printed in German found near the scene of the attack in Dortmund suggested a possible Islamist motive for the attack.

One of the letters, published by Bild, starts with the Islamic phrase “In the Name of God, the Most Gracious, the Most Merciful” and demands the withdrawal of German Tornado surveillance planes from Syria and the closure of the U.S. military air base in Ramstein, Germany.

“As of now, all infidel actors, singers, athletes and all prominent personalities in Germany and other crusader nations are on a death list of the Islamic State,” the letter said.

Bild said prosecutors were also investigating two additional suspects in the attack, one a neo-Nazi sympathiser and another from a radical leftist group.