"Provisional post-mortem of Usmani suggests that 90 grams blood clot was found in his brain. He even had a heart-attack. His heart has been kept separately for further medical examination," he said.
The report also reveals that Usmani's body bore no marks of injury.
Usmani is the brother of Afzal Usmani, 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.
According to his family, Usmani died because of torture in custody. A CID inquiry has been ordered into the mysterious death of Faiz by Ajith Parasnis, Maharashtra DGP.
According to Usmani's son Azeem, his father was "pressurised during interrogation. They are to blame for his death."
"When the police came to take him, they said that they will just interrogate him. He had gone alone. So we don't know what the police questioned him about. I don't know to what extent he was tortured, that he got so tensed. They came back in an hour saying he was unwell, he had vomited and admitted to Sion Hospital," said Rubaida, Usmani's wife.
The Mumbai police however are denying the charges.