Want to re-investiaon fir

hello sir im law student and i've practising as junior and matter is going on-- that is fir lodge u/s ipc-279,337,338,184 person drive a bike and death show under the said fir but afterthat we have go to cause of action place and inquire about themselves and find out a eye witness who has say he was shown the said accident and car slams into another ,it crashes into it with great force and so that such person fall on the road side and cause death. my question is 1) how to deal the fir and how to re - investigaon the said fir. 2) the said vehicle bike under custody of police station so release the said vehicle or not ? for issue of re investigation. 3) or any other point of veiw reliable under th e law. plz advise and reply