DORTMUND: German federal prosecutors said Thursday they had cleared the sole suspect in custody for a bomb attack against the Borussia Dortmund football team bus of involvement.

The announcement marked a setback for investigators, who described the three blasts late Tuesday as a 鈥渢errorist鈥� act and said they are focusing on suspects in the 鈥淚slamist spectrum.鈥�

鈥淭he investigation has not found evidence that the suspect took part in the attack,鈥� the prosecutors office said in a brief statement.

It said it was nevertheless seeking an arrest warrant for a 26-year-old Iraqi national, identified only as Abdul Beset A., for alleged ties to Daesh.

Investigators had zeroed in on two suspects believed to belong to the large extremist scene in the Ruhr region, after three identical letters claiming responsibility for the attack were found at the scene. Only Abdul Beset A. was detained.

The letter demanded that Germany stop its Tornado reconnaissance missions in the international anti-Daesh coalition and close the US air base at Ramstein in western Germany.

The daily Bild newspaper said that police had placed Abdul Beset A. under surveillance for several months and believed, based on tapped telephone conversations, that he might be hiding explosives in his flat.

However, a raid on his home Wednesday drew a blank, the report said, adding that investigators were still pursuing leads in the extreme-right and far-left scenes.

Bild also quoted a security expert, Peter Neumann, raising doubts about a Daesh link to the letter found at the scene because some of its formulations were atypical for the group.

Even as the probe appeared to be in the preliminary stages, Dortmund officials criticized the decision to play its postponed Champions League match just 24 hours after the attack, with the perpetrator or perpetrators still at large.

The roadside blasts left Dortmund鈥檚 Spanish international Marc Bartra and a policeman injured, with the bombs 鈥渃ontaining metal pieces鈥� detonating minutes after the team bus set off to a planned game against Monaco.

The quarter-final, first leg match was held in Dortmund 24 hours later in a packed stadium with tight security, with Monaco winning 3-2.

Dortmund coach Thomas Tuchel angrily accused UEFA of treating the bomb attack as if a 鈥渂eer can鈥� had been thrown and claimed they were informed by text message that they would have to play their Champions League game.

Former German international Lothar Matthaeus said it was 鈥渋rresponsible鈥� to get the players to go through with the game so soon after the attack.

鈥淔rom what I heard from team sources, many players didn鈥檛 want to play today. But UEFA put on pressure and politicians urged Borussia Dortmund to counter terror,鈥� he told Sky News, referring to the European football federation.

Chancellor Angela Merkel said Wednesday she was 鈥渉orrified鈥� by the 鈥渞epugnant act,鈥� which Dortmund city鈥檚 police chief described as a 鈥渢argeted attack鈥� against the team.

Germany has been on high alert since a series of militant attacks last year, including a Christmas market truck rampage in Berlin.

Merkel said Thursday that 鈥渉ighly varying鈥� laws for each region under Germany鈥檚 federalist system were undermining the country鈥檚 fight against terrorism.

She cited differing policies on surveillance as one weakness and called for their harmonization, in an interview with the Funke media group.

鈥淲e know that we are threatened like many other countries and will do everything in our power to ensure security in freedom for our citizens, in close consultation between the federal government and the states,鈥� she said.

Before the match began Wednesday, fans chanted 鈥淏artra! Bartra!,鈥� in support of the injured defender.

Dortmund鈥檚 chief executive Hans-Joachim Watze vowed that his side would 鈥減lay not only for ourselves today. We will play for everyone... we want to show that terror and hate can never determine our actions.鈥�

Interior Minister Thomas de Maiziere, who attended the match, said the 鈥渇ascination鈥� surrounding football drove terrorists to try to disrupt it.

鈥淭hat鈥檚 why it鈥檚 right that we do as much as we can to protect it, and not allow criminals to take the fascination away from us.鈥�