According to his friend, Naseem Siddiqui, the postmortem of Usmani revealed that he died of hypertension followed by heart attack and then brain hemorrhage.
"This is what the family has been conveyed orally, we will get a copy of the autopsy report later. We will complete his funeral today (Monday)," said Siddiqui.
Meanwhile, the family members of Usmani cried foul and said that Usmani died due to "torture and pressure tactics".
Usmani is the brother of Afzal Usmani, the main accused in the 2008 Gujarat blasts and an Indian Mujahideen member who is currently lodged in jail.
Usmani was picked up by the Chembur Crime Branch on Jul 17 and was then rushed to a hospital in Sion after he complained of ill health.
He died in the morning at 1.30 am when he was undergoing treatment for hypertension and hemiparesis in the Lokmanya Tilak Hospital, said doctors.