Hameed Dabholkar said the CBI should complete the probe and arrest the "main perpetrators".
Hameed Dabholkar, son of late anti-superstition activist Narendra Dabholkar, said on Tuesday that it was "painful" that even after seven years, the probe into his father's murder has not reached a conclusion.
Dr Narendra Dabholkar, founder of the Maharashtra Andhashraddha Nirmulan Samiti (Maharashtra Eradication of Superstition Committee), was shot dead by two motorcycle-borne assailants in Pune on August 20, 2013.
Dr Hameed Dabholkar, his son, said the Central Bureau of Investigation (CBI) should complete the probe and arrest the "main perpetrators".
"On Thursday, it will be seven years to the incident. It is painful that after seven years, a reputed investigation agency such as the CBI has not been able to complete the probe," he told reporters.
The agency has filed charge sheets against Virendra Tawde, Sharad Kalaskar, Sachin Andure, Sanjeev Punalekar and Vikram Bhave, but not against remaining arrested accused -- Amol Kale, Amit Degvekar and Rajesh Bangera, he said.
"In the investigation so far, it has not yet made it clear who were the real perpetrators. The CBI should find them. Otherwise the threat to the freedom of speech of writers, rationalists and journalists will remain," he added.
"When Dr Dabholkar was murdered, the Congress-NCP government was in power. When Comrade Govind Pansare was murdered (in 2015), there was BJP-Shiv Sena government and now there is a coalition government of Shiv Sena, Congress and NCP," he said.
All the parties like to use the epithet "progressive" for Maharashtra, Hameed Dabholkar said.
"In fact, the 'progressive Maharashtra of Shahu-Phule and Ambedkar' is a mandatory phrase in the speeches. But unfortunately, the investigation of the murder of a person who gave up his entire life to create a society based on progressive thoughts is not completed after seven years," he said.
So far the CBI has arrested eight people including Andure and Kalaskar, the duo who allegedly shot Dabholkar.
One of the accused, Punalekar, is out on bail.
In March this year, the CBI claimed to have recovered a firearm from a creek near Thane, which it said could have been used in the killing.