It was later known that Adolf Prokop had been a Stasi officer, employed as an officer in special service (OibE), and that several referees, including Bernd Stumpf, had been Stasi informants. But there is no proof that referees stood under direct orders from the Stasi and no document has been found in the archives that gave the Stasi a mandate to bribe referees. The benefit of controlling important matches in Western Europe, gift to wives and other forms of patronage, might have put indirect pressure on referees to take preventive action, in so called pre-emptive obedience. In order to pursue an international career, a referee would need a travel permit, confirmed by the Stasi. The German Football Association (DFB) has concluded that "it emerged after the political transition that Dynamo, as the favorite club of Stasi chief Erich Mielke, received many benefits and in case of doubt, mild pressure was applied in its favor". Adolf Prokop protests against having manipulated matches. He was never banned from refereeing. He points out that top teams are viewed with scepticism and claims to have never received threatening letters from angry fans. Adolf Prokop was still invited to nostalgia matches for the East German national football team in the 2010s.