It's been a hard week for Borussia Dortmund, but one that has seen the entire football world united behind them after their team bus suffered a shocking bomb attack before their meeting with Monaco in the Champions League on Tuesday.
German newspaper Tagesspiegel said on its website on Friday it had received an anonymous far-right email claiming responsibility for Tuesday's attack.
"The pain, the panic, and the uncertainty of not knowing what was going on, nor how long it would last. they were the longest and hardest 15 minutes of my life", the Spain global wrote on Instagram on Friday.
Bartra was hurt by flying glass and had to undergo a wrist operation, taking him out of action for four weeks.
Wolfsburg, which was in danger of being overtaken by the visiting side, stayed one point above the relegation zone due to Augsburg and Mainz both winning.
Borussia Dortmund players display a Marc Bartra shirt as they celebrate after the Bundesliga match at home to Eintracht Frankfurt April 15, 2017.
One of the letters, published by Bild, starts with the Islamic phrase "In the Name of God, the Most Gracious, the Most Merciful" and demands the withdrawal of German Tornado surveillance planes from Syria and the closure of the United States military air base in Ramstein, Germany.
Dortmund lost 3-2 to Monaco when the game was played on Wednesday, less than 24 hours later than originally scheduled.
However, according to reports in Germany, police investigators suspect that the letters were written to pin blame on radical Islamists and that the attack could have been carried out by political extremists or violent football fans.
Borussia Dortmund bounced back from the trauma of the bomb attack on their team bus and their Champions League defeat in midweek to earn an emotion-filled 3-1 home win over Eintracht Frankfurt in the Bundesliga on Saturday.
One suspect - an Iraqi man - was arrested over the alleged Islamist link but has since been cleared of involvement.
Spanish defender Marc Bartra suffered injuries to his wrist and arm caused by shattered glass from the explosion, which happened about 90 minutes before Dortmund were due to play Monaco at the nearby Westfalenstadion.
Watzke revealed that he had left each player with the choice of whether to play or not.
"We have the letter claiming responsibility", said Frauke Koehler, a spokeswoman for federal prosecutors.