Four days after the bomb attack Tuesday on the bus of the Borussia Dortmund (BVB) football club there is still no clear information regarding either the identity of the attacker(s) or motive.

On Tuesday evening at 7:15 p.m. three bombs loaded with steel pins exploded as the bus set off from the team’s hotel to the local stadium in Dortmund. The bombs were hidden behind a hedge and detonated by remote control. They had a destructive power of up to a hundred yards.

BVB was due to play AS Monaco in a Champions League quarter-final when the bombs struck.

Despite its reinforced panels the bus was badly damaged. The Spanish BVB defender Marc Bartra suffered an injury to his wrist and was operated on the same evening. One metal splinter only just missed the BVB players and drilled into the headrest of a seat. “We are lucky nothing worse happened,” Frauke Köhler, spokeswoman for the prosecutor generals’ office declared.

Only one day after the attack she declared that the attack had a “terrorist background” and that the federal prosecutor’s office had taken over the investigation. Germany’s highest state investigation authority is responsible for terrorist offences.

Investigators from the Federal Prosecutor’s Office and the Federal Criminal Police Office arrested two men from North Rhine-Westphalia on Wednesday night. Special Forces stormed the apartments of a 25-year-old Iraqi in Wuppertal and a German living in the small neighbouring village of Unna. Both have Islamic backgrounds.

One man was arrested, but on Thursday it was announced that the investigations “had so far no evidence that the accused had been involved in the attack”. The second suspect was not even arrested.

Investigators reported on Friday that they had “serious doubts” that the attack had been carried out by Islamists as initially reported. They pointed to the suspicious character of letters found at the scene, which appeared to be written in an attempt to misdirect investigators.

Shortly after the detonation, police officers near the scene of the crime found three identical letters professing to the attack, presumed to have been left behind by the perpetrator(s). The letter stated that the “Islamic State” (IS) was behind the bombing, but there is much evidence indicating that the claim of responsibility was deliberately aimed at sending a false signal.

This would be the first ever such letter left by an IS attacker at a crime scene. The letter, written on computer, fails to include any oath of allegiance to IS leader Abu Bakr al-Baghdadi, the IS logo or any other religious formula. The 14-line letter is also written in apparently deliberately faulty German, which gives rise to even more doubts. The letter includes obvious spelling mistakes but at the same time the author has no problem with much more difficult grammatical formulations.

Another new feature of the letter is the way in which the author(s) directly address chancellor Angela Merkel (CDU): “But apparently Merkel, you do not care a jot for your little filthy subjects. Your Tornadoes still fly above the ground of the caliphate to assassinate Muslims.”

The letter specifically demands the withdrawal of German Tornado fighter planes from Syria and the closure of the Ramstein Air Base, the largest American military base in Germany. Until that is accomplished, “all disbelieving actors, singers, athletes and celebrities in Germany and other crusader countries everywhere are on the death list of the Islamic state.”

According to the Nordrhein Westfalen Interior Minister, Ralf Jäger (SPD), the three bombs were professionally constructed. Jäger said that, based on the letters found, there was a high risk of further attacks, and he announced additional personnel would safeguard the public and all major events.

A second letter published on the Indymedia website, is very likely to be spurious. “Neither the content or language point to a left-wing background, so we have already deleted it shortly after its release” the platform said. The investigating authorities drew a similar conclusion. The internet post said that the bus had been attacked as a “symbol for the policy of the BVB”, which had not done enough to combat racists, Nazis and right-wing populists.

Bearing in mind that the BVB spends €300,000 per year for so-called “Nazi Prevention”—it subsidizes trips for pupils and fans to concentration camp memorials such as Dachau or Sachsenhausen—both the investigators and many football fans consider it possible that right-wing hooligans are behind the attack on the BVB.

The football club is currently taking legal action against a group of fans from the club, including many ultra-right thugs, who were involved in provocations against a rival team from Leipzig in February.

One week after the February incident police stopped several busses containing members of the group “0231 Riot” (0231 is telephone dialing code for Dortmund). The “0231” thugs were on their way to an away game of the BVB in Darmstadt. The group recruits almost exclusively from the extreme right-wing scene.

After the BVB banned the group from its stadium, police in Dortmund found graffiti threatening the life of BVB boss Hans-Joachim Watzke.

The investigating authorities also believe that violent supporters of the RB Leipzig—the club at the receiving end of the violence in February—could be behind the terror attack. The demands contained in the alleged IS note calling for the withdrawal of German Tornado aircraft from Syria and the closure of the US air base in Ramstein are almost identical to the demands of the far-right political movement Legida, which is based in Leipzig.

Despite the attack, the football match between BVB and AS Monaco went ahead one day later. Immediately after the explosion, the Union of European Football Associations (UEFA) decided to allow the match to take place. The Dortmund coach, Thomas Tuchel, expressed his disquiet with the decision at a press conference immediately after the match. “The dates were planned in advance and we have to work,” he said, but the team would have liked more time to deal with the shock of the bombing.

Soon after the terror attack and after the game a veritable storm of solidarity messages for BVB and criticism of the UEFA decision hit the internet and social networks.

Players had also expressed their unease after the game. BVB defender Sokratis said: “The UEFA needs to understand that we are not animals. We are people with a family and children at home. I’m glad all the players and supporters are alive.”

In an interview with a Norwegian TV station, the BVB player Nuri Sahin said: “To be honest I did not think about football until I was on the pitch in the second half.” His teammate, Matthias Ginter, told ruhrnachrichten: “Nobody wanted to play today.”

BVB head Watzke concurred, however, he had agreed with the UEFA on Tuesday in order to send a “signal against terror.” He was supported by Chancellor Merkel, who called him personally Wednesday morning.

The fact that the game was put back a day created many problems for hundreds of Monaco fans who had planned their return home after the match. Many Dortmund fans responded by offering overnight accommodation in their homes for the stranded fans.