German state prosecutors on Wednesday said they assume the attack on the Borussia Dortmund bus had a "terrorist motive." 

"We found several letters of responsibility. One of them suggested a possible Islamist motive. Another demands the withdrawal of German military in Syria and the closure of the US facility at Ramstein," said Frauke Köhler, a spokeswoman for the federal prosecutor's office.

"We currently have two suspects from the Islamist milieu. One of the two has been arrested," she added. Police noted that metal rods had been placed in the explosives to cause significant harm. A rod had pierced one of the bus' seats, according to authorities.

Earlier, security officials had said they were investigating an alleged message from the far-left Antifa claiming responsibility for the attack, but Köhler said investigators expressed "considerable doubts" that it was genuine.

Three explosive devices detonated near Borussia Dortmund's bus on Tuesday as it was leaving the team's hotel on its way to the Champions League quarterfinal match with French side Monaco.

Spanish defender Marc Bartra was injured in the explosion and underwent surgery for injuries to his wrist and lower arm, but has since thanked his team mates and fans for their support in a message published on social media platform Instagram.

Click here to read more: 'Champions League: After bus attack, Dortmund look to refocus on Monaco'

"As you can see, I am doing much better. Thank you everybody for all your support and your messages! All my strength to my team mates, supporters and fans, and to BVB for tonight's match," Bartra said, referring to the team by its initials.

The match was rescheduled for Wednesday evening.

'Despicable act'

German Chancellor Angela Merkel condemned the "despicable act." She called Borussia Dortmund CEO Hans-Joachim Watzke personally to offer her solidarity.

"Our thoughts today are with the players … with Borussia Dortmund, with the fans. And we hope that it can be a peaceful and good game," she said in a statement.

Ralf Jäger, interior minister for the state of North Rhine-Westphalia, where Dortmund is located, told the press that police were "investigating every possible avenue" in relation to the attack, whether it be far-left, far-right or Islamist in nature.

Jäger called on the public to "give the police some time," to do their job properly, adding that it was possible more suspects would be found.

'We will not give in to terror'

Borussia Dortmund managing director Hans-Joachim Watzke said the team will not cave into terror following an attack on their bus on Tuesday.

Click here to read more: 'Humanity prevails as football fans stand together in Dortmund'

"I've just been in the changing rooms and called on the team to show everyone out there that we will not give in to terror," Watzke said in a statement.

"We're not just playing tonight for ourselves. We're playing for everyone, whether they're Borussia Dortmund, Bayer or Schalke. We want to show that terror and hate shall never determine our actions," he added.

ls, es/kms (AP, AFP, dpa, Reuters)