The unexploded bomb, defused by experts from the National Security Guard and a bomb squad, was encased in a small bucket-shaped cement container and contained ammonium nitrate and fuel oil as the explosive material, senior police officials said.

From the outside, the device would look like an inverted bucket shaped cement block on account of a smooth cement finish at the top. Pre-cast cement, embedded with nuts and bolts acted as the missile material contained for the bomb and a micro-chip acted as a timer, a senior police official said.

From an analysis of the bomb sites and forensic evidence, police have summarised that all devices were made under common supervision. Karnataka director general of police R Srikumar described the bombs as being “professionally assembled”.

“Only at two bomb sites — Adugodi and the Vittal Mallya road circle - tiffin-box encasings for the devices were found. At all other places, including the unexploded bomb, found near the Big Bazaar on Saturday had plastic casings,” sources said.

Police have found leads to show that the bombs were assembled in Bangalore itself. Giving credence to this theory is the finding of local English newspapers in the packing of the bombs. One of the latest newspapers bears the date of June 27, another dates back to March, sources involved with the probe stated.

Police are trying from eyewitness accounts to arrive at a possible time when the bombs could have been planted around the city. A security guard at a store at one of the sites has told police investigators that at around 7 am on Friday morning he saw a “plastic bucket-like” object near the trash area while he was watering plants in the shop yard. Bangalore city police officials who questioned people at the site of the discovery of the unexploded bomb have reported that people saw it lying there on the night of July 24 — nearly 12 hours before the blast.

“It was used as a stool to sit on by some local street vendors and an alarm was raised on Saturday morning after some one noticed wires emerging from the plastic encasing,” a police official said.