The Delhi Police has questioned over 50 people spotted near the CRPF school in Rohini before a blast occurred there last month. Those rounded up include a local resident who dropped a cigarette butt at the spot few minutes before the explosion.

 "Whether the blast took off due to the cigarette butt is yet to established as we still don't know the exact composition of the explosive," a senior police officer said on Saturday, adding that forensic reports from the National Security Guard (NSG) and Forensic Science Laboratory (FSL) are still awaited.

 He also said it is yet to be confirmed if the white powdery material found at the spot after the blast was industrial waste.  The police said they are yet to find any leads in the probe into the explosion.

 According to the officer, over 50 people were spotted near the school in the CCTV footage of 10 days preceding the blast, but none of them have been identified as suspects.

 A man walking with his dog was found to be the last person at the spot, which is usually used for dumping waste and public urinal.

He was seen smoking a cigarette, after which he threw the butt and left.

A few minutes later, the blast took place, according to the CCTV footage, the officer said. The man, a resident of Prashant Vihar, has been questioned and the police do not suspect his involvement in the blast, he said.

 The strong blast ripped through the wall of the school in Rohini's Prashant Vihar area on October 20. While no one was hurt in the explosion, it damaged the hoardings of nearby shops and window panes of some vehicles parked in the area.

 The police suspected that the explosion was caused by an IED (improvised explosive device) that was planted to give a message to the government authorities.

 They also suspected that the explosive, wrapped in a brown coloured packet, was made using a mixture of three chemicals -- hydrogen peroxide, borate and nitrate -- weighing more than 2 kg.

 Another senior officer privy to the investigation said, "There are multiple theories regarding people seen near the spot and speculations about industrial waste but we are yet reach a conclusion as we don't know its (the bomb's) composition."  "Our further probe depends on the forensic report from NSG and FSL," the officer added.

 He said as of now ten days of CCTV footage has been thoroughly checked, in which over 50 people were spotted near the spot.

"All have been verified and no one was found as a suspect," he added. The officer also said the police have still not ruled out the possibility of a terror angle into the case.

 After the blast, the Delhi Police forensic teams and experts from the NSG collected samples from the spot. A day after the incident, a team of Central Reserve Police Force from Pune also visited the spot and collected samples.

 A pro-Khalistani group had posted the blast's CCTV footage on social media claiming the explosion was in retaliation to the alleged "targeting" of pro-Khalistan separatists by Indian agents.