A powerful military explosive — one often used by Islamic militants fighting in India's part of Kashmir — was used in last week's Mumbai train bombings, investigators said Monday.

Indian officials have linked the attacks that killed 182 people to Pakistan, and the use of RDX explosives is a hallmark of the militants, who get at least a degree of support from Islamabad in their fight to wrest the predominantly Muslim Kashmir from India.

"The explosive used was a mixture of ammonium nitrate, RDX and fuel oil," said K.P. Raghuvanshi, who leads the anti-terrorist squad investigating the July 11 bombings.

CountryWatch: India

Raghuvanshi declined to speculate on who carried out the attack, saying only that investigators had fanned out across India to track down leads.

But Mumbai Police Chief A.N. Roy said earlier Monday that: "We believe the breakthrough will come soon."

RDX, one of the most powerful high explosives, is widely used for military purposes, usually combined with other explosives, oils, or waxes. It is also a key component of artillery rounds, mines, and plastic explosives.

The low-volatility explosive has some civilian applications including fireworks and demolition kits, and is used occasionally in rat poison.

India's suspicions have prompted New Delhi to suspend a two-year peace process with its nuclear rival and demand a "firm commitment" from Pakistan on reining in the militants.

Prime Minister Manmohan Singh said Sunday that 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.