"Investigation in this case is at the preliminary stage since NIA has recently taken over the probe from Goa Police's SIT," National Investigation Agency (NIA) has said in an application moved in a court in Panaji.
The agency said they will require the custodial interrogation of the accused to unearth larger conspiracy behind the blast that killed two persons.
NIA took over the probe almost two months after the October 16 explosion in the commercial city. The deceased were allegedly ferrying explosives in a scooter when they went off.
The probe by a Special Investigation Team (SIT) had led to the arrest of four persons linked to the group, Sanatan Sanstha.
They have been accused of hatching a conspiracy to trigger serial blasts across the tourist state on Diwali eve.
The plan went haywire after the explosives went off prematurely, alerting the police, the SIT had said.
The deceased were members of the Sanstha.
group has said the victims were its members, but denied involvement in the blast. In its reply opposing the bail application of one of the accused, Dilip Mangaonkar, NIA told the court that a lot of ground still remains to be covered.
A lot of things like collection of evidence, their connection with the accused and scrutiny of bank accounts remain to be done, the NIA said.
The agency has said some of the accused had used copies of bogus election photo identity cards and obtained SIM cards from different agents of mobile companies for communication prior to the blast.
The NIA has said the conspiracy was hatched on August 25- 26, 2009 in which improvised explosive devices (IEDs) were prepared.
"After having prepared circuits, test explosions were conducted in the hills at Talaulim village in Ponda taluka," the application said.
During investigations, the remnants of the test explosion and other electronic articles connected with the IEDs were recovered, it said.
Interrogation of the accused had led to the seizure of articles like PCB (printed circuit board) and electronic components linked to the crime, the NIA said.