By Tilman Blasshofer and Matthias Inverardi
KARLSRUHE/DUESSELDORF, Germany (Reuters) - German authorities arrested a suspected Islamist on Wednesday in connection with what Chancellor Angela Merkel called a "despicable" attack on a bus carrying players of one of the country's top soccer teams.
Merkel said she was appalled by Tuesday evening's incident, in which three explosions went off as the Borussia Dortmund bus made its way to a match. Spanish defender Marc Bartra suffered injuries to his arm and wrist.
The attack forced a 24-hour postponement of the team's high-profile clash with AS Monaco, which turned out to be a pulsating encounter that the visitors won 3-2.
"I encouraged everyone to take the game seriously but football is not the most important thing in the world," Borussia Dortmund coach Thomas Tuchel said after the game.
Police stepped up security for the rescheduled match and for a second Champions League quarter-final on Wednesday between Bayern Munich and Real Madrid.
An exit from the Dortmund stadium was briefly closed as police examined suspicious packages, but quickly re-opened.
Ralf Jaeger, interior minister in the state of North Rhine-Westphalia which includes Dortmund, said the investigation was looking "in all directions", and it was unclear if one or several attackers were involved.
A spokeswoman for the Federal Prosecutor's Office, which handles probes into suspected terrorism, said the explosive devices had contained strips of metal.
Spokeswoman Frauke Koehler said investigators had found three letters near the scene, all with the same content suggesting a possible Islamist motive.
She said the letters referred to the use of Tornado reconnaissance planes in Syria, which Germany has deployed as part of the military campaign against Islamic State, and also called for the closure of the U.S. military base at Ramstein in western Germany.
Investigators had identified two suspects from the "Islamist scene", searched their apartments and detained one man, she said.
The blasts smashed windows on the bus carrying the players to the stadium. Bartra was operated on for a broken bone in his right wrist and shrapnel in his arm, a team spokesman said.
"We all agree that we are dealing with a despicable act," Merkel said at a business event in the western town of Allendorf.
"We were all appalled yesterday when we heard about the attack on the bus with the players of BVB in Dortmund," she added, praising Dortmund fans for offering accommodation to AS Monaco fans after the postponement.
Bartra, 26, joined Dortmund for 8 million euros (£7 million) last year from Barcelona. He has made 12 appearances for the Spanish national team.
The blasts revived memories of Islamist militant attacks in Paris in November 2015 whose targets included a stadium where France were playing Germany in a soccer friendly.
Security is among the issues at the heart of Germany's parliamentary election on Sept. 24, a tight battle in which Merkel is running for a fourth term. In December, a Tunisian man killed 12 people when he ploughed a truck through a Berlin Christmas market.
(Additional reporting by Joseph Nasr, Paul Carrel, Anneli Palmen and Jens Hack; Editing by Mark Trevelyan)