The special teams formed to track down the culprits have been making efforts to end the elusive run of Santhosh, Pattabi and Surendran, police said.
Arul Prasad, an employee of a prominent University here, in whose house the country made bomb went off on May 3, was being questioned, but has not revealed much over the incident, police said.
Mohan Ram, another history sheeter, was undergoing treatment at a Government hospital and is yet to be interrogated, police said.
While Arul, who had fled from hospital, was arrested on May 4, the other three are still at large.