Sources said that the police are examining the hard disk of a closed circuit television camera (CCTV) recorder at the BJP office and that footage has captured a person with suspicious movements.
Police sources said that 2 kg of grade one explosives were used in the blast. Intelligence sources have said the bomb was an Improvised Explosive Device or IED, but it is not yet clear how it was triggered.
Police said the bomb appeared to have been placed on a motorcycle with the Tamil Nadu registration number - TN 22R 3769, which was parked outside the BJP office around 10:15 am. The registration number has turned out to be fake.
A large fire followed the explosion. Several vehicles including a van of the State Reserve Police were gutted. Windows of buildings nearby were shattered.
Karnataka's Home Minister R Ashok, who is also the deputy chief minister, has described the blast as "terrorist activity" and said BJP workers were the target.
Union Home minister Sushil Kumar Shinde has said the Centre will begin an inquiry into the incident. When asked if this could be described as a terror attack, he said that would be known only "after the probe was completed."
A team from the National Investigation Agency (NIA), which has expertise in handling terror attacks, has inspected the site. A team from the National Security Guard's (NSG) bomb data squad is also at the site.
State elections are scheduled in Karnataka for May 5. Today is the last day for candidates to file their nomination papers for the state elections, so a police van was posted outside the BJP office in Malleshwaram.
Moments after the blast, Congress spokesperson Shakeel Ahmed tweeted, "If the blast near BJP's office in Bangalore is a terror attack, it will certainly help the BJP politically on the eve of election." The BJP condemned the remarks and said they were "unfortunate and condemnable".
The Congress has distanced itself from Mr Ahmed's comments. Minister of State for Home RPN Singh, said, "In matters relating to terror attacks, no political capital should be extracted."