Delhi Police has identified six persons as suspects who visited the blast site prior to the incident days after a bomb explosion ripped through the wall of a Central Reserves Public Force (CRPF) school in Rohini Sector 14.

According to police sources, the police are currently awaiting forensic reports to determine the exact nature of the blast. He added that they have questioned around 100 people so far in connection with the case.

Police are now trying to locate the six persons who were seen in the blast site at different time intervals before the explosion according to the source. Earlier, security and intelligence agencies were also probing the role of Khalistani supporters in the blast after a purported Telegram post was being circulated on social media which claimed that the blast was carried out in retaliation to the "targeting" of pro-Khalistan separatists allegedly by Indian agents.

"The police have scanned over 72 hours of CCTV footage, which led them to focus on six persons seen near the CRPF school," the source said, adding that their identities are yet to be verified.

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.

Earlier, CCTV footage of a suspect from the night before the incident was found and more than two-dozen people who were spotted near the site just before the explosion were questioned. A man in a white t-shirt was also identified by the police through the CCTV footage.

An officer privy to the investigation said the police have questioned around 100 people, including locals, shopkeepers and security guards, but they have not got any major breakthrough yet. The police are also trying to ascertain the exact composition of the bomb suspected to have been planted one foot below the ground near the school wall.

A senior police officer said forensic teams and experts from the National Security Guard (NSG) have collected samples from the spot and a report is awaited. It is suspected that the bomb wrapped in a brown coloured packet was made using a mixture of three chemicals -- hydrogen peroxide, borate and nitrate -- weighing more than 2 kg.

A day after the incident, a team of Central Reserve Police Force (CRPF) from Pune also visited the spot and collected samples. On Thursday, a team from the National Investigation Agency (NIA) held a meeting with Delhi Police officials in Rohini.