Starting in 1958, Grünenthal received hundreds of complaint letters and reports by doctors and pharmacists. In those reports Thalidomide was repeatedly connected to (often severe) nerve damage and many other health problems. For example, in August 1958 Grünenthal received a letter from Dr. Kreideweiss in Dormagen (a small town in North-Rhine Westphalia). The doctor reported on difficulties in walking after taking Contegan (Thalidomide) for a long period of time.
Instead of urgently and properly investigating those reports and complaints, Grünenthal kept reassuring doctors and pharmacists that their drug was safe. Each time, in their repeated responses to all the early warnings, they claimed that this was the first time complaints like these were brought to their attention.
Source: Anklageschrift (indictment) from 1967, today archived at the National Archives of North Rhine-Westphalia in Duisburg, Germany (Rheinland Division, Gerichte Rep. 139, No. 1–396), pp. 65ff.