The explosive found in the parking lot of the Delhi high court on May 25 was not a dry run for September 7 terror strike but an operation undone by faulty equipment and damp conditions.
Investigators believe that the May 25 explosion and September 7 blast — the death toll climbed to 13 on Thursday — are the work of the same group, who learnt from the failed attempt.
The improvised explosive devices (IED) used are similar — both had shrapnel and timer devices, most likely mobile phones.
The May 25 attempt failed because of defective detonators and dampness, say forensics.
“More than one detonator was used in the May 25 IED. It can also be safely said the IED was bigger than the one used on Wednesday,” said an investigator, who refused to be identified as he is not authorised to speak to the media.
The battery didn't send enough charge for the detonator to set off the blast, burning the explosive, the investigator said.
"That left the ammonium nitrate-based IED unexploded. It also didn't help that the IED was packed in a bag and the area where it was left was damp." There is a distinct possibility that the person tasked with planting the bomb got nervous or ran out of time and couldn't assemble the bomb properly.
But no such mistakes were made while planting the explosive at the busy gate 5 of the high court. The IED was packed in a briefcase not a bag - protecting the explosive against rain and dampness.
And, a briefcase - commonly used by lawyers and litigants - would hardly arouse suspicion in a court complex.
"This time, the terrorists used pentaerythritol tetranitrate (PETN) which is a far better nitrate explosive than ammonium nitrate. It is a commercially available explosive used in mining," said a source, who didn't wish to be identified.