German police investigating the explosion which rocked Borussia Dortmund's team coach on Tuesday evening are treating the incident as a "targeted attack".

The Bundesliga outfit's Champions League clash at home to AS Monaco was postponed after the bus was caught up in a blast, leaving defender Marc Bartra needing hospital treatment for minor injuries that he sustained when its windows shattered.

Bartra broke his wrist during the attack and is due to undergo surgery to address the injury and remove debris from his arm.

Police chief Gregor Lange has also confirmed that a letter making claims of responsibility was discovered at the scene of the explosion, which happened near to the team's hotel, but is yet to disclose its specific contents.

"As of now, it is still unclear as to what motive was behind the attack. We cannot exclude anything," he told reporters. "We want to thank Borussia Dortmund, the fire department and everyone involved for their professionalism throughout this evening.

"The stadium guests reacted fantastically - it made the situation a lot easier for us. We are of course preparing for a major police operation for tomorrow."

The investigating officers are yet to establish how the explosive devices were detonated, but Lange said that his team are keeping an open mind about the possibility that the perpetrator was lying in wait.

"Three explosions detonating at the exact moment the bus passed by suggest a sophisticated expertise in both bomb building and detonation - perhaps using a mobile phone or a garage door-opening device," added police spokesman Gunnar Wortmann.

"We will be working throughout the night to discover who was behind this attack. All police and vehicles in Dortmund have been mobilised."

The authorities are hunting a car with foreign registration plate in connection with the incident.

Dortmund's Champions League match against Monaco has been rescheduled for 5.45pm (BST) on Wednesday.