Termination clauses

Dear sir, I have resigned in my company on 11th may 2016.There is a notice period of 30 days. I have choosen the relieving day on 14th may 2016.They have not relieved me.They have tried to retain me. As per my boss, he is saying that there is some internal investigation is running for you.we can't releive as i have put up my resignation on 11th may in system.My resignation was not accepted.I have discussed with hr.She is also using the same line.She says that you will have to survive your notice period for 30 days because there is some investigation is running.9th may 2016 is my releving date.Hr says that if investigation will not be complete till 9th june.We will releive you. My question is that they can terminate me during the notice period. Hr doesn't share the reason on mail. If internal investigation is running why they are not mailing me. Hr is also saying that extend ur releving date.