Forensic examinations have confirmed that the explosive used in Tuesday's serial blasts on Mumbai's suburban trains was not the deadly RDX but could have been dynamite or ammonium nitrate.

The revelation may change the focus of the probe on the alleged role of Laskhar-e-Tayiba in the blasts, police said.

Laboratory tests performed on nearly a dozen samples collected from seven blast sites have confirmed that the explosive used was not RDX, a top Mumbai police official told PTI, but refused to reveal the nature of the explosive.

Intelligence sources, however, said that it could be plain dynamite, which was commonly used in India for blasting rocks and in mines. They have also not ruled out the possibility of the use of chemical ammonium nitrate in the explosives.

Mumbai police sources said that the use of commonly-employed explosive material like dynamite or ammonium nitrate in the present serial bomb blasts not only dilutes the theory of involvement of bigger terrorist groups like the LeT but also gives credence to the possible involvement of less-sophisticated local groups.

Police said they are likely to ask forensic experts to perform confirmatory tests to entirely rule out RDX, and added that the focus of investigations could change with these results and they will now have to concentrate on local groups and issues related to it.

"Looking at the meticulous execution of the blasts, one thing is sure that the preparations for the blasts were made much in advance and the executors could have been only waiting for an opportunity," a top Mumbai police official said.

The probe will now have to focus on whether the incidents like the recent disturbances at Bhiwandi in neighbouring Thane district, or the crackdown on SIMI groups in the past few months has anything to do with the blasts, he added.

Meanwhile, police said the interrogation of detained SIMI cadre was still on and no definite clue has come up in the probe so far.

"We are waiting to check the discrepancies, if any, in their statements and may hold another round of interrogation thereafter," police added.