Police in Dortmund say investigators “are working on the assumption” that the explosions that hit team bus were caused by “serious explosive devices”.
And German publication Bild claim bombs were hidden in a roadside hedge near the exit of the team hotel.
Defender Marc Barta was taken to hospital with a hand injury but is said to be ok and has been sent home.
Dortmund have since confirmed Bartra is undergoing surgery to remove debris from his right arm.
Dortmund goalkeeper Roman Buerki says the team bus had just pulled out of a hotel driveway at about 7:15 p.m. when it was rocked by an explosion.
Buerki told Swiss daily newspaper Blick on Tuesday night that “there was a huge bang, literally an explosion” that sent glass flying about.
Police have said there were three explosions near the bus.
The Swiss keeper says he was sitting in the last row of the bus, next to Spanish defender Bartra. The team earlier identified Bartra as the only player injured.
Buerki says Bartra was hit by shards of glass from the broken back window.
He says players ducked for cover, wondering if there would be more explosions.
Buerki says police were at the scene quickly.
He told Blick: “We’re all shocked. Nobody thought about a football match in the minutes after that.”
The game will now take place tomorrow evening after confirmation from Dortmund.
The Dortmund team bus was on its way to Westfalenstadion but was stopped around 10 km away after an explosion shattered the windows and injured Bartra.
A police spokesman said: “Various windows were broken due to the force of the explosions.”
“Shortly after the departure of the Borussia Dortmund team bus from the hotel to the stadium there was an incident. The bus has been damaged in two places,” Dortmund said in a statement.
“One person has been injured and is in the hospital. At this point we will inform as soon as we know more.”
Police have revealed that they discovered a letter near the scene of the crime and are trying to authenticate it – although they have not disclosed the contents of the letter.
They say the letter contains a claim of responsibility but haven’t revealed who.
Dortmund’s CEO Hans-Joachim Watzke released a statement after the game had been postponed.
“The players are in shock,” he said. “We must get through this, but it will not be easy.
“Borussia Dortmund come closer together in extreme situations and I’m sure the players will feel that.”
Dortmund have moved to help Monaco fans find accommodation for the night by advertising #bedforawayfans on their Twitter account.
