Police said the bomb appeared to have been placed on a stolen motorcycle with a Tamil Nadu licence plate, which turned out to be fake.
Police said footage from a closed circuit television or CCTV camera at the BJP office shows a man parking the motorcycle about 15 minutes before the blast.
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.
Police said 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 are still attempting to determine its signature.
A large fire followed the explosion. Several vehicles including a van of the State Reserve Police were gutted. Windows of buildings nearby were shattered. (Pics)
Security was high in the area with elections scheduled to be held on May 5 and the van was full of cops, 11 of whom were injured, though none critically. Today was the last day for candidates to file their nomination papers.
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.
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 remark and said it was "unfortunate and condemnable". But Mr Ahmed tweeted again, "Karnataka HM R Ashok says blast near our office was aimed to kill BJP leaders due to elections. This will also generate some sympathy." (Read)
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."