Syed's health suddenly deteriorated on Saturday evening, following which he was shifted to the Government Medical College and Hospital (GMCH) where he died one and half hours after admission, Jail Superintendent Rani Bhosle told media on Sunday night.
"The exact cause of his death will be known after post-mortem which will be conducted Monday in the presence of his relatives who have arrived in the city Sunday," she said, adding that Syed's body will be handed over to his relatives after autopsy.
"As per preliminary information and symptoms, he could have died of a cardiac arrest," Bhosle said.
Syed, his wife Fehmida and third conspirator Ashrat Ansari were convicted by a POTA (Prevention of Terrorist Activity) court in 2009 of planting powerful bombs in two taxis which exploded at the iconic Gateway of India and Zaveri Bazaar on August 25, 2003, leaving 52 people dead and 244 injured.