Meanwhile, players and other clubs passed on their messages of support, as the football family came together.
The German Federal Prosecutor's Office said three similar letters claiming responsibility found near the site of the attack before Dortmund's Champions League match at home to Monaco on Tuesday night made an Islamic extremist motive possible.
Amid safety concerns for 65,800 sell-out crowd and players alike, the match was rearranged for the following day, Wednesday, April 12 and Spaniard Bartra underwent a successful operation on his injuries that night.
Police said the three "serious explosive devices" had been concealed in a hedge on the route to the ground.
Dortmund says Bartra will not be able to play for several weeks.
But Koehler said investigators were still evaluating the credibility of the note.
Spanish defender Marc Bartra, injured in the attack, said he was "doing much better" after surgery on his wrist.
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One of two claims of responsibility for the attack - found in three identical letters left at the scene - mentioned a demand for the withdrawal of German Tornado jets from Syria and the closure of the US Air Force's Ramstein air base in Germany.
The chancellor, who takes a keen interest in the fortunes of Germany's national soccer team, also praised Dortmund fans for taking in AS Monaco supporters overnight so they could stay on for the postponed match. "They will concentrate, they are professional players".
"In France, we know this", said Monaco supporter Pierre Calmon, 50, from Toulouse, who was attending the match with his son, 16-year-old Thomas. Monaco's fans were thanked by Dortmund for chanting their hosts name repeatedly as the initial reports started to reach the stadium on Tuesday evening. "We're preparing a major police presence for [the rescheduled game] and will do everything humanly possible to ensure the match can take place safely".
The match was put back to Wednesday as security was ratcheted up around Dortmund and in Munich where Bayern Munich will take on Real Madrid.
Dortmund is one of Germany's most popular soccer teams and a regular contender for the Bundesliga title, which it last won in 2012.
"We are assuming that they were a targeted attack against the Dortmund team", said the western German city's police chief Gregor Lange.
Team flags flapped throughout the city Wednesday and dozens of fans gathered in a wind-swept parking lot outside the team's training complex in the morning, where four young women drew "You'll Never Walk Alone" in black markers.
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