Three explosions hit the Borussia Dortmund team bus ahead of their Champions League quarter-final with Monaco.
The chief executive of a Dortmund, Hans-Joachim Watzke, said police informed him that the explosives that went off near the team bus were hidden by the exit of a hotel and detonated as the bus passed.
The explosions happened as Borussia Dortmund was leaving the L'Arrivee Hotel and Spa, a 12-minute drive from the stadium, Signal Iduna Park, for a match against Monaco.
Defender Marc Bartra was injured in the explosion and was taken to hospital, the team said on Twitter, wishing the player a quick recovery.
Tuesday night's match was cancelled following the explosion and has been rescheduled for Wednesday.
Dortmund police spokeswoman Nina Vogt said investigators do not yet know the source of the explosions.
There are no indications, however, that the three explosions were part of an organised terror attack, national news agency DPA reported, citing unnamed security sources.
'No element' points in the direction of such an assault, said the German agency, in line with police who have so far also avoided describing it as a terror attack.
Police confirmed that there was no immediate danger to anyone in and around their stadium.
Police said in a statement they were working on the assumption that the blasts were caused by 'serious explosive devices', which may have been hidden in a hedge near a car park.
They didn't elaborate on the possible nature of the devices or say who might have planted them ahead of first-leg match between Borussia Dortmund and Monaco.
Explosive devices were believed to have been placed in a bush by the road along which the bus was travelling, according to German newspaper Bild.
'The explosive devices were placed outside the bus. Several windows were broken,' police spokesman Gunnar Wortmann was quoted as saying.
In a statement, Dortmund police said earlier that they could not say 'exactly what the explosion was or exactly where something exploded'.
'According to what is currently known, the windows of the bus were (entirely or partly) smashed and one person was injured,' Dortmund police said, adding that the incident happened in Hoechsten, located outside the city.
Several windows were smashed in the explosion, and Bartra, a Spanish national, was the only player to be taken to hospital.
Borussia Dortmund goalkeeper Roman Buerki said the team bus had just pulled out of the hotel driveway when it was rocked by an explosion.
Buerki told Swiss daily Blick that 'there was a huge bang, literally an explosion' that sent glass flying about.
The Swiss keeper said he was sitting in the last row of the bus, next to Bartra. He said Bartra was hit by shards of glass from the broken back window.
Buerki added that players ducked for cover, wondering if there would be more explosions. He said police were at the scene quickly.
The goalkeeper told Blick: 'We're all shocked. Nobody thought about a football match in the minutes after that.'
Bartra, the only player injured in the explosion, joined the club from Barcelona last year, and his former club were quick to wish him well.
'All of our support to @MarcBartra, @BVB and their fans,' Barcelona tweeted in support of their former player.
Bartra, 26, joined Dortmund for eight million euros last year from Barcelona, after coming through the Catalan club's youth system.
He has made 12 appearances for the Spanish national team.
Borussia Dortmund's managing director Hans-Joachim Watzke was quoted telling Sky TV: 'The whole team is in a state of shock.'
Monaco striker Radamel Falcao also sent his best wishes to the player and his club, tweeting: 'I'm sorry for everything that has happened. We are all well. I wish a speedy recovery to Marc Bartra.'
Dortmund also released a statement explaining what happened.
'Shortly after the departure of the Borussia Dortmund team bus from the hotel to the stadium there was an incident,' Dortmund said.
'The bus has been damaged in two places. One person has been injured and is in the hospital. At this point we will inform as soon as we know more.'
In the immediate aftermath of the explosions, Dortmund said there was 'no cause for concern' for those at the stadium, and added the club were in close contact with the emergency services and UEFA.
AS Monaco goalkeeper Danijel Subasic told Croatian daily newspaper 24sata: 'We are currently in the stadium, in a safe place, but the feeling's horrible.'
An announcement was made inside the stadium with a message put on the big screen informing supporters of the incident.
Police in Dortmund said in a statement: 'In the run-up to the Champions League game between BVB (Dortmund) and AS Monaco, there was just after 1900 tonight an explosion close to the BVB team bus.'
Following the announcement that the match had been postponed, Dortmund police tweeted: 'To reassure stadium visitors and relatives: There are currently no indications of a threat to visitors in the stadium.'