The blast on August 8, 1993 was triggered using RDX. It brought down the multi-storeyed RSS office at Chetput in Chennai, claiming 11 lives.
Ahmed had allegedly procured the explosive material for assembling the bomb and provided shelter to the other accused, officials said.
The agency had announced a reward of Rs 10 lakh for providing credible information about Ahmed, one of the main accused in the case.
The agency was entrusted with the probe in 1993 and filed a chargesheet against 18 persons under the stringent provisions of the Indian Penal Code, Explosive Substances Act and Terrorist and Destructive Activities (Prevention) Act (TADA).
In 2007, a TADA court in Chennai in 2007 convicted 11 persons and awarded life terms to three after a 12-year trial.
The agency continued its hunt for Ahmed -- who did not face trial as he could not be arrested -- right through.
After the trial in 2007, the special court acquitted four persons, including S A Basha, founder of the banned Al Umma, for lack of evidence against them.
Immam Ali, a suspected ISI agent who had escaped from custody in Madurai, was gunned down in Bangalore in an encounter with the police on September 29, 2002.
Another accused, Jihad Committee founder Palani Baba, was hacked to death by suspected RSS sympathisers on January 28, 1997.
Of the 431 witnesses, 224 were examined during the course of the trial which commenced on August 7, 1995.
The chargesheet was filed on June 8, 1994.