However there are some offences which require the victim to explicitly request prosecution (Antragsdelikt), other cases may be dropped due to "small guilt" or "non-importance" or the complainant may be told to prosecute on his own (common in defamation cases unless a VIP is involved). Prosecutors are authorised to undertake investigations themselves or can request the police to do so and the police are obliged to carry out such requests. Moreover, prosecutors can order witnesses and expert witnesses to testify before them and can even ask an investigating judge (the Ermittlungsrichter) to interrogate witnesses or to examine the evidence.