Indian officials have linked last week's train bombings, which killed 182 people, to neighboring archrival Pakistan, which has denied having a hand in the blasts.
Last week, Prime Minister Manmohan Singh told a news conference that the bombers were "supported by elements across the border," a reference to Pakistan.
"If the acts of terrorism are not controlled it is exceedingly difficult for any government to carry forward what may be called the normalization and the peace process," he said.
The use of RDX explosives is a hallmark of bombings by Islamic militant groups, some of them Pakistan-based, who have long been fighting to wrest part of Muslim-majority Kashmir from predominantly Hindu India.
India accuses mostly Islamic Pakistan of materially aiding the rebels. Pakistan says it only offers them moral and diplomatic support. Both countries claim all of Kashmir, which is divided between the nuclear-armed countries and has triggered two wars between them since their independence from Britain in 1947.
India's suspicions have prompted New Delhi to suspend a two-year peace process with Pakistan, and to demand a "firm commitment" from Islamabad on reining in the militants.
"The explosive used was a mixture of ammonium nitrate, RDX and fuel oil," K.P. Raghuvanshi, who leads the anti-terrorist squad investigating the bombings, told reporters.
Raghuvanshi declined to speculate on who might have carried out the train attacks, saying only that investigators had fanned out across India to track down leads.
Bombay Police Chief A. N. Roy had said earlier Monday that officials believe the breakthrough in the probe will come soon.
Prime Minister Manmohan Singh on Sunday said India's two-year-old peace process with Pakistan cannot move forward if terrorism "aided and abetted from outside" continues to claim lives in India, the Press Trust of India news agency reported.
Last Friday, Bombay Police Commissioner A.N. Roy said a man known only as Rahil was the third person being sought in connection with the blasts. The photos ofwere released Thursday. The nationalities of the three men have not been given.
Investigators are casting a wide net in their hunt for the assailants, scrutinizing a Pakistan-based Islamic militant network, the Lashkar-e-Tayyaba, along with smaller homegrown groups.
An Indian Home Ministry official said that among the leads being followed by investigators was the possible involvement of the outlawed Students Islamic Movement of India, which may have been aided by Pakistan's Inter-Services Intelligence agency.
The investigation also spread to neighboring Nepal where police said Friday they had arrested two Pakistanis in connection with the seizure of RDX explosives in the country's capital, Katmandu, in 2001. They said the two men are also being investigated for links to the Bombay blasts, CBS News' Ranjan Gupta reported.