The charge sheet was filed before the Principal District and Sessions Judge Muhammed Ibrahimkutty by the Assistant Commissioner of Police George Koshy, official sources said.
There were a total of four accused, stated to be members of the 'Base Movement' outfit, in the case. One person turned an approver.
They had been charged under sections of the Unlawful Activities (Prevention) Act and also for conspiracy.
A low-intensity Improvised Explosive Device kept under an unsued jeep exploded at the collectorate cum-district court's complex on June 15, 2016 morning, injuring a 61-year-old man.