The 'whodunnit' question has been hounding investigators from the beginning as the blast in a flower market in Mehrauli in south Delhi on September 27, came exactly two weeks after the serial blasts in the national capital.

The bomb, which was dropped in the market by two motorcyclists, who masked themselves with helmets, also did not have "signature components" of the explosives used by suspected Indian Mujahideen terrorists.

Three persons were killed and 21 were injured in the blast.

Suspicion fell on Bangladeshi nationals and local criminal gangs as there were at least three mysterious blasts in south Delhi this year. Two Bangladeshis were also arrested in this connection earlier.

"We have questioned several people. The investigations are still on. We are confident that we will crack the case soon," a senior police official said.

Similar modus operandi was used in Maharashtra and Gujarat, where bombs were placed by motorcycle-borne men to trigger blasts three days after the Mehrauli explosion.

"We are interacting with our counterparts in both states," the official said.