Borussia Dortmund travelled to Monaco for the second leg of their Champions League quarterfinal amid heightened security after the attack on their team bus last week.
The Federal Prosecutor's office in Germany is investigating after three explosions hit the Dortmund team bus as it drove to their home stadium for the first leg last week.
Defender Marc Bartra was injured, along with a police officer escorting the bus, and the game was postponed for 24 hours.
Monaco won the rearranged fixture 3-2.
Ahead of the return match, Dortmund players were escorted to the airport by police, WDR reported, as security was tightened.
Their flight was delayed by 15 minutes as a police helicopter monitored the runway.
Speaking to SID, Monaco's minister of the interior, Patrice Cellario, said security in the Principality had been heightened.
"Dortmund did not ask for anything, but we will increase the security measures for the squad and the bus," he said.
Local paper Ruhr Nachrichten reported that German investigators had reconstructed the scene of the attack as well as taking samples of earth from the area of the blasts for analysis.
Speaking to dpa, a spokesperson for the Federal Prosecutor's office refused to comment on reports that those responsible for the attack could have been right-wing extremists or connected to a gambling syndicate.
"We do not comment on media reports," the spokesman said, adding that he did not want to elaborate on whether the explosions were still regarded as a terror attack.