All India Muslim Majlis-e-Mushawarat president Zafarul Islam Khan alleged that there was no evidence against him to file a chargesheet.
"Since his arrest seven months back, investigative agencies could not find any proof to file a charge sheet against him. Therefore, a conspiracy was hatched to get him killed by a criminal who was already facing murder charges," Khan told reporters here.
He said they have also written to National Human Rights Commission (NHRC) demanding a probe into the killing.
"How can a murder take place in a high security cell of a jail? We demand a judicial enquiry by a sitting high court judge into this," Khan said.
Siddiqui was in Pune's high security Yerawada jail for his alleged role in planting a bomb outside Shrimant Dagdusheth Halwai Ganapti temple in Pune and was a named accused in the Bangalore stadium blast.
The Maharashtra government had ordered a CID probe into the death of Siddiqui.