Death in Police Custody

One of my relative is a Police Officer, under his custody a woman died, initially her relatives were not ready for a post mortem saying that she has died a natural death and there is no need for it, but since it had happened in Police Custody, the police insisted on Post mortem and finally a Post Mortem was conducted by a team of 2 Doctors, which was fully videographed,the doctors concluded that "no marks of any external or internal injury found in the body and No abnormality detected but cause of death is uncertain therefore viscera is being preserved". The viscera report also came totally normal and nothing wrong was found in it. Apart from it there was also an inquiry by a Magistrate into it, in which the statements of various people and of the Doctors were recorded and the Magistrate also exonerated the police and concluded that she died of natural death. Now the relatives of deceased have filed a petition before the CJM u/s 156/3 and the magistrate ordered it to be treated as complaint case. Though i think that police has a very strong case, i would like to know some opinion on it.