A forensic report received by the Maharashtra ATS suggested that there was presence of RDX powder in the third floor apartment in Habib Building at Byculla, sources said.
The triple blasts last year at Zaveri Bazar, Opera House and Dadar claimed 27 lives and left more than 130 people injured.
ATS has so far arrested four IM operatives Haroon Naik, Naqi Ahmed Wasi Ahmed Sheikh, Nadeem Akhtar Ashfaq Sheikh and Kanwar Nain Wazeer Chand Patrija and invoked the stringent provisions of Maharashtra Control of Organised Crime Act against them.
According to the ATS officials, Naqi had helped IM's top operative Yasin Bhatkal, who is also mastermind of the attacks, to rent the third floor apartment in Habib Building last year.
The arrested perpetrators had assembled the explosives and prepared bombs in the flat and later used them to carry out terror attacks.
Forensic experts, who had collected samples from the terror sites, had opined that ammonium nitrate mixed with fuel oil was used in the explosives while digital timers were used to execute the blasts.
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As the probe progressed, ATS officials had stumbled upon the Byculla flat where the suspects had hidden and assembled the explosives.
Forensic experts subsequently had collected various articles including a computer and clothes of the inmates of the Byculla room for examination.
The samples collected from the flat suggested that there was RDX in the flat, but since it was not found at the terror sites, it indicates that the explosive material was not used for the preparation of bombs, sources added. PTI VM GK HKS