Jude Bellingham moved to Dortmund from Birmingham City in 2020The German Football Association is investigating England midfielder Jude Bellingham's comments about referee Felix Zwayer after Borussia Dortmund's 3-2 defeat by Bayern Munich.
Bellingham, 18, questioned Zwayer's appointment, referring to his six-month ban for a match-fixing scandal in 2005.
Zwayer rejected Dortmund's appeals for a penalty before giving their rivals a spot-kick for a Mats Hummels handball.
Robert Lewandowski scored the penalty to secure victory for Bayern.
Speaking to Viaplay,external-link Bellingham said: "You give a referee, that has match fixed before, the biggest game in Germany. What do you expect?"
Bayern's win in Der Klassiker on Saturday saw them move four points clear of Dortmund at the top of the Bundesliga.
"For me, it wasn't [a penalty]. He [Hummels] is not even looking at the ball and he's fighting to get it and it hits him," added Bellingham.
"You can look at a lot of the decisions in the game."
The DFB control committee have written to Bellingham and asked him to comment on his statements "as soon as possible".
Once Bellingham's response has been evaluated, the control committee will decide what action to take.
Dortmund are "100%" behind Bellingham, said sports director Michael Zorc.
"It was a very emotional situation, he was very disappointed and only named known facts," Zorc told Ruhr News.
Dortmund chief executive Hans-Joachim Watzke added: "To be clear, Jude was not spreading lies, but what happened in the past.
"This statement shouldn't have been, but I don't see anything untrue there. I don't expect that to have any consequences for Jude. "
