Malappuram district police sources said some steel pieces were found near the blast site and it is suspected that a 'pressure cooker' was used to trigger it.
Security was tightened in Tamil Nadu on Friday following reports that six members of the terror outfit LeT had infiltrated the state by sea.
The couple-Rajan, 47, and his wife Ambili, 40, natives of Nellimoodu at nearby Neyyattinkara- succumbed to burns at the government medical college hospital in Thiruvananthapuram after they sustained multiple organ failure on Monday.
Fearing violent response from the mob present outside the Police Club and court complex, several rings of security were thrown around Islam who was seen wearing a helmet. Before producing him in the court, he was interrogated by top officials of the Special Investigation Team probing the case.
The reports suggested six members of the terror outfit infiltrated into the state by sea from Sri Lanka and moved to different cities, including Coimbatore, according to the police.
The director general of police said everyone in the force should have to perform their duties irrespective of their religion and politics.
The letter alleged that some of the recent articles written by the award-winning writer amounted to "misleading" the Muslim youth. It also warned that Ramanunni would meet the same fate as that of professor T J Joseph, whose right arm was chopped off by the members of a radical Muslim outfit for allegedly hurting their religious sentiments through a question paper he had set.
He was arrested based on evidence police gathered during the process of investigation into the case.
Prohibitory orders were imposed in Delhi and parts of Karnataka. Police kept tight vigil in Kerala. In Gujarat, 50 people were arrested for Thursday's violence.