The Federal Prosecutor's office in Germany says there has been no evidence to link a suspect who was detained in connection to the three explosions that hit the Borussia Dortmund team bus to the attack.

An investigation is underway after the explosions rocked the Dortmund bus as it drove to their home stadium for a Champions League match against Monaco, injuring defender Marc Bartra and causing the game to be postponed for 24 hours. Monaco won the rearranged game 3-2 on Wednesday evening.

Prosecutors said in a statement that the man detained, a 26-year-old Iraqi named Abdul Beset A. (his surname wasn't provided due to German privacy laws) was "strongly suspected of being involved in Iraq as a member of the foreign terrorist group 'Islamic State' [IS]."

The statement added: "The accused was arrested [on Wednesday] in connection with the attack on the Borussia Dortmund team coach. The investigations have so far shown no evidence that the accused participated in the attack.

"However, the accused is suspected of joining Islamic State at the latest by the end of 2014 in Iraq. According to our intelligence, he was in command of a unit of about 10 people. The job of his unit was to prepare abductions, extortions and also killings.

"He is also said to have fought for unification. In March 2015 he travelled to Turkey and from there to Germany in early 2016. From Germany the accused continued to maintain contacts with members of 'IS'.''

An arrest warrant has been issued and a judge will decide if he is kept in custody for a further 24 hours for questioning.

Authorities also searched the home of a second suspected Islamic extremist but prosecutors said on Thursday there were no indications he was involved in the attack.

Police had said on Wednesday that a letter found near the scene suggested a possible Islamist extremist motive and that two suspects from the "Islamist spectrum" were at the centre of the investigation. Their flats were searched and one person was detained.

Police in Dortmund after the blasts near the BVB team bus. Photo by Maja Hitij/Bongarts/Getty Images

The letter found at the scene demands the withdrawal of German Tornado reconnaissance jets from Turkey and the closure of the United States' Ramstein Air Base in Germany.

"It is to be assumed that there is a terror background," a Federal Prosecutor's office statement released on Wednesday said. "The motives are yet unclear.

"There were three claims of responsibility near the scene of the attack. They all had the same content, and an Islamist background can't be ruled out at this point."

Tobias Plate, a spokesman for Germany's interior ministry, said notes claiming responsibility at the scene had not been a feature of previous Islamist attacks.

The Federal Prosecutor's office statement said the explosive devices used "were armed with metal pins, one of them drilling itself into one of the headrests of a seat. We can be thankful that nothing more serious happened."

It said the explosive power was "up to 100 metres."

Experts say the letter's mix of correct, complicated German and obvious mistakes raises the possibility that the letter is a red herring intended to place blame on Islamic extremists.

Police say the metal-packed devices used in the attack were sophisticated and would have required detailed knowledge of explosives and detonators, suggesting possible military training.

Information from The Associated Press was used in this report.