"The MDMA is investigating the larger conspiracy. From CBI reports, it doesn't seem that there is much headway. So this enquiry could be endless," a bench comprising Justices Ranjan Gogoi and R Banumathi said.
The bench fixed the matter for hearing on January 24.
The apex court had on November 14 asked the government to respond to a plea of the convict, A G Perarivalan, seeking suspension of his sentence till the CBI concluded its probe into the conspiracy behind the making of the belt bomb that killed Gandhi and several others.
The court asked the Centre to clarify its stand on the plea by Perarivalan, whose death sentence was earlier commuted to life term by the apex court.
In his plea, Perarivalan has told the court that he was held guilty for supplying two nine-volt batteries which were allegedly used in the improvised explosive device (IED) that killed Gandhi on May 21, 1991, but the probe by CBI-led MDMA relating to the IED was still going on.
The convict has said he has spent 26 years behind bars in connection with the case, while the Tamil Nadu government has already decided to set him free by remitting the remaining portion of his sentence.
Rajiv Gandhi was assassinated on the night of May 21, 1991 at Sriperumbudur in Tamil Nadu by a woman suicide bomber, identified as Dhanu, at an election rally. Fourteen others, including Dhanu herself, were also killed.
This was perhaps the first case of suicide bombing which had claimed the life of a high-profile global leader.
The Centre had earlier filed in a sealed cover a report on the investigation being carried out on the conspiracy aspect behind the making of the bomb that had killed Gandhi.
The apex court had on August 17 asked the Centre and CBI to apprise it of the probe being carried out on the conspiracy aspect behind the making of bomb after Perarivalan had claimed that this aspect was not being probed properly.
The apex court had on February 18, 2014 commuted the death sentence of Perarivalan to life imprisonment, along with two other condemned prisoners -- Santhan and Murugan -- on grounds of a delay of 11 years in deciding their mercy pleas by the Centre.