"We received an anonymous complaint on Monday," Günther Feld, a senior prosecutor for Cologne, confirmed to German sports agency SID on Tuesday. In the complaint, Podolski is accused of assault on Ballack, and TV coverage of the match, seen by an estimated 10 million people, is cited as evidence.
The complaint says that a footballer is a public role model, and that “such an act carried out before millions” is unacceptable.
According to the law, a facial slap does count as assault even if the victim is not injured and the assailant apologises afterwards.
Feld told reporters that state prosecutors are currently checking who is responsible for the anonymous complaint, and are deciding whether they will conduct an investigation into Podolski's actions.
Express reported that since the incident happened abroad, it comes under the jurisdiction of the victim's hometown – Ballack lives in London - but other news agencies are reporting that the Cologne authorities have turned the matter over to state prosecutors in Munich, where Podolski lives.
“Because the game took place in Cardiff, it is now to be handled in the area of Podolski's place of residence," Feld told reporters.
Media reports said that Podolski and Ballack had a tactical disagreement, or that Podolski was directed to put in more of an effort by his captain, to which Podolski allegedly responded, “Run yourself, you ass,” Express reported.
He vehemently denies using such language, but has since offered an apology, calling himself an “idiot” and accepting a €5,000 fine from the German Football Federation (DFB).
Neither the DFB nor Podolski's club Bayern Munich have commented on the possible court case.