Delhi Police questioned more than 50 people spotted near the CRPF school in Rohini before a blast took place there last month. Among those gathered was a local man who threw a cigarette butt at the scene minutes before the explosion.

“It is yet to be ascertained whether the blast was triggered by cigarette butt as we still do not 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 the Forensic Science (FSL) is still awaited.

He also said it was not yet confirmed whether the white powdery material found at the scene after the explosion was industrial waste. Police said they have yet to find any leads in the blast investigation.

According to the officer, more than 50 people were spotted near the school on CCTV footage 10 days before the blast, but none of them were identified as suspects.

A man walking his dog was found to be the last person at the site, which is usually used for littering and public urinals.

He was seen smoking a cigarette, then threw some snot and left.

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

The powerful blast tore through the wall of the school in Prashant Vihar area of ​​Rohini on October 20. Although no one was injured in the explosion, it damaged the signs of nearby shops and the windows of some vehicles parked in the area.

Police suspected that the blast was caused by an IED (Improvised Explosive Device) that was planted to send a message to government authorities.

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

Another senior officer close to the investigation said: “There are several theories about people seen nearby and speculation about industrial waste, but we are yet to come to a conclusion as we do not know the composition of it (the bomb). “Our further investigation depends on the forensic report of NSG and FSL,” the officer added.

He said that so far ten days of CCTV footage has been thoroughly checked in which more than 50 people were spotted near the spot.

“Everything was checked and no one was found suspicious,” he added. The officer also said that the police have yet to rule out the possibility of a terror angle in the case.

After the blast, forensic teams of Delhi Police and experts from 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 posted CCTV footage of the blast on social media, claiming the blast was in retaliation to the alleged “targeting” of pro-Khalistan separatists by Indian agents.