A German police investigation into who carried out the bomb attack on the Borussia Dortmund football team's bus is reportedly focusing on both Islamic extremists and anti-fascist groups.

German prosecutors say a letter found near the scene of the Dortmund team bus blasts suggests a possible Islamic extremist motive for the attack, and one suspect has been taken into custody.

"We also have explosive detection dogs in use and made a decision to search the scene from the air".

German Tornado reconnaissance planes are taking part in a non-combat role.

Fans at the 80,000-capacity Signal Iduna Park were told to stay there until it was safe to leave.

"One can only be relieved that the consequences were not worse", he said, praising Dortmund fans for offering accommodation to AS Monaco fans after the postponement.

"We're investigating in all directions".

State interior minister for North Rhine-Westphalia Ralf Jaeger, speaking in Dusseldorf earlier, said the number of police had been increased significantly in Dortmund ahead of the rescheduled game.

It's believed an attacker was "watching and waiting" to set off the explosives, which were intended for the team. Police are working on confirming the authenticity of the letter.

Officials said a letter was found near the scene and that two suspects were sought, one of whom was detained.

German Chancellor Angela Merkel has condemned the attack on the Bundesliga team as "repugnant" as the investigation continues.

Defender Marc Bartra, a Spanish global, was taken to hospital after being injured by shattered glass from the coach's windows. Spanish defender Marc Bartra suffered injuries to his arm and wrist. He will not play Wednesday.

Manchester United said it was "sending support" to Dortmund. "The prevailing mood in the Black and Yellow camp seems to be to play the match tomorrow for Marc".

"However, we expect and I am confident that the team will do its best and deliver a spectacle in the Champions League this evening", he said.

"Thanks for all your messages!"

Dortmund's players were escorted by gun-wielding officers throughout the day and armed police have been patrolling the grounds as fans arrive for the quarter-final first leg match.