Investigators were struggling for definite clues hours after a powerful bomb went off outside the Delhi high court on Wednesday morning.
Officials were not even sure of the nature of explosive used in the blast even as Secretary Internal Security in the home ministry UK Bansal said terrorists may have used hard-to-detect plastic-based explosive PETN, or pentaerythritol trinitrate, hidden in a briefcase.
Sharing details of the "preliminary" investigations, Bansal told reporters that traces of PETN have been recovered from the blast site.
"But this is the result of preliminary investigation. Rigorous investigations are in progress," Bansal said, indicating that the explosive traces found were not conclusive.
The investigation is being led by a 20-member team of the National Investigation Agency (NIA) being assisted by the National Security Guard (NSG) and Delhi Police.
The impact of the blast was so huge that a two-feet wide and one-foot deep crater was formed just outside the Gate No.5 of the court complex, where around 300 people were gathered.
The explosive quantity was estimated to be about two kilograms and police claimed to have recovered the metal panel of the briefcase measuring 1.25 feet by one foot, in which the explosive was kept.
Police have released sketches two suspected attackers -- one in his mid-20s and another 50 years old. The sketches were drawn based on descriptions given by eye-witnesses.
A Delhi Police team, meanwhile, cordoned off the blast site and shifted all the injured to various hospital. At least 11 people were killed and 76 injured in the blast.
A police official said investigators were not ruling out the possibility that the attackers, or at least one among them, might have injured himself or been killed in the blast.
"It may be possible that the bomb planter could not escape from the spot before the explosion took place. We are verifying and tracking all the injured and dead," said the police officer.
The official also said they were working on different lines of investigations.
The other line of probe the investigators were following is that the attacker may have left the briefcase near the gate and left. There was a long queue of people waiting to get their entry passes to enter the court at the time of blast at 10.15 am.
The investigators have also not found any timer. If a timer had been used, the bomber would have had ample time to escape.
The NIA is also verifying the authenticity of an email received by two TV channels in which Pakistan-based Harkat-ul-Jehadi Islami has claimed responsibility for the blast.