German investigators Thursday focused their probe into three explosions that rocked the Borussia Dortmund football team bus on a sole "Islamist" suspect in custody, after the side defiantly returned to the pitch.
The roadside blasts left Dortmund's Spanish international Marc Bartra and a policeman injured, with the bombs "containing metal pieces" detonating minutes after the team bus set off to a planned Champions League game against Monaco on Tuesday night.
As fans of the two sides expressed solidarity in a packed stadium, the quarter-final, first leg match was held in the western German city 24 hours later, with Monaco claiming a 3-2 win.
Investigators were chasing a suspected "terrorist link" to the blasts, after three identical letters were found at the scene.
"An Islamist background appears to be possible," federal prosecutor's office spokeswoman Frauke Koehler said, noting the letter demanded that Germany withdraw its deployment of Tornado reconnaissance missions in the anti-IS international coalition and close the US air base in the western German town of Ramstein.
The investigation is focused on two suspects "from the Islamist spectrum" she said, adding that both their homes had been raided and one man was detained.
Investigators have until late Thursday to build a case against him sufficient for an arrest warrant, or let him go.
Local media identified the suspects as a 28-year-old German and a 25-year-old Iraqi, with the latter in custody.
At least one of the two could have been close to the scene at the time of the explosion, they said, quoting unnamed security sources who nevertheless urged caution on linking the individuals to the assault.
- 'Repugnant act' -
Chancellor Angela Merkel said Wednesday she was "horrified" by the "repugnant act", which Dortmund city's police chief Gregor Lange described as a "targeted attack" against the team, also known as BVB.
Germany has been on high alert since a series of jihadist attacks last year, including a Christmas market assault in Berlin.
Tuesday's explosives detonated minutes after the Dortmund team bus pulled away from the squad's hotel.
Bartra underwent surgery on a broken wrist after he was hit by flying glass, Dortmund president Reinhard Rauball said.
A policeman, who was on a motorcycle escorting the team bus, suffered trauma from the noise of the blasts, which shattered the bus windows.
"The bus turned on to the main road, when there was a huge noise -- a big explosion," Dortmund's Swiss goalkeeper Roman Burki told Swiss media.
"After the bang, we all crouched down in the bus. We did not know if more would come." Some players hurled themselves to the ground, he said.
As the squad geared up for kick-off Wednesday, Dortmund's chief executive Hans-Joachim Watze vowed that his side "will play not only for ourselves today. We will play for everyone... we want to show that terror and hate can never determine our actions".
Before the match began, fans chanted "Bartra! Bartra!", in support for the defender who was hurt.
- 'Irresponsible' -
Interior Minister Thomas de Maiziere, who attended Dortmund's match, said the "fascination" surrounding football drove terrorists to try to disrupt it.
"That's why it's right that we do as much as we can to protect it, and not allow criminals to take the fascination away from us."
But former German international Lothar Matthaeus said it was "irresponsible" to get the players to go through with the game so soon after the attack.
"From what I heard from team sources, many players didn't want to play today. But UEFA put on pressure and politicians urged Borussia Dortmund to counter terror," he told Sky news channel, referring to the European football federation.