Days after a low-intensity blast took place near the Israeli embassy here, Delhi Police have registered an FIR against "unknown" persons, officials said on Saturday.

The FIR was registered on Friday night under Section 3 (punishment for causing an explosion likely to endanger life or property) of the Explosive Substances Act and Section 427 (mischief causing damage to the amount of fifty rupees) of the Indian Penal Code at Tughlak Road police station based on a complaint lodged by a police officer, an official said.

The blast occurred on Tuesday evening in the area between the boundary walls of a house on plot number 4 -- Nanda's House -- and the Central Hindi Training Institute on plot number 2A on Prithviraj Road. The area has bushes, plants and trees and no CCTV camera.

It is behind the Israeli embassy on Dr APJ Abdul Kalam Road, running parallel to Prithviraj Road.

No one was injured in the blast. Police had found an "abusive" letter addressed to the Israeli ambassador near the blast site.

According to sources, the one-page letter written in English is suspected to be related to an organisation called Sir Allah Resistance and mentions words like "Zionists", "Palestine" and "Gaza".

On Friday, police told PTI that they were planning to register an FIR as they had found "crucial evidence" pointing to a conspiracy to threaten the Israeli envoy.

Another official said on Saturday that the case might be handed over to the Special Cell to dig out the deeper conspiracy.

Delhi Police officials, who are investigating the case, have scanned footage of multiple CCTV cameras installed around the blast site and spotted a suspect who had come from Jamia Nagar in an autorickshaw.

Police have questioned multiple autorickshaw drivers, including the one who brought the suspect to the spot before the blast took place.

However, police are yet to trace the accused.

The officials said police had recovered a broken dial of a watch and some steel bearings from the blast site but it was too early to say if they were used in the explosion.

Investigating officials are awaiting reports from the National Security Guard's (NSG) laboratory, where samples collected from the spot have been sent to ascertain the components used in the explosion.

Police have recorded statements of a dozen people who claimed to have heard the loud noise of the blast.

The statements reveal that the witnesses spotted a vehicle which broke down near the blast site.