Murder

My friend murdered recently who worked as a cab driver in vishakapatnam. he murdered very cruelly with knief and pouring acid on him while he was in drivingby four people, they ran away with that car.finally those persons were caught by police in that area. police enquired and they told in enquiry that we murdered for car only.police also says they are murder for car only and not for every one .we have doubt on cab owner.but we dont have any proofs to prove.any one of his friend (victim) did not say any thing.police aslo said that we cannot not do anything because you dont have proofs to prove cab owner is done all this by them.i and all my friends,neighbours feel he is good enough,innocent.we personally enquired all his collegeas but they dint reveal even small one. we strongly believe that cab owner did something all this by them.we dont like to left this as mystery. i need your advise how we proceed further with the police .and through court .what we have to do and follow to get justice... sir sir please help me .. thaking you sir