Belal Hossain Patwari, 48, was arrested in Khagrachhari’s Matiranga Upazila around 2:30am on Thursday, said Matiranga Police Station OC Mainuddin Khan.
Patwari worked as a personal assistant to the slain Upazila chairman, locals said.
Ekram’s supporters set Patwari’s Anandapur house in Phulgazi on fire on Monday, insisting that he was involved in the plan of the murder.
OC Khan said Feni Model Police Station OC Mahbub Morshed told him that Patwari was one of the masterminds of the killing.
Ekramul Haque, president of AL Phulgazi unit, was killed in a brazen daylight attack at Feni Sadar on May 20.
Assailants blocked his car, opened fire on the vehicle and set it ablaze with the chairman trapped inside.
So far 22 people have been arrested on charges of their involvement in the murder.
Police arrested BNP’s affiliate Tanti Dal leader Mahtabuddin Ahmed Chowdhury Minar, an accused in the murder case filed by Ekram’s brother, from Dhaka on Tuesday.
Phulgazi Awami League’s general secretary Jahidul Chowdury Jihad, said to be the mastermind behind the killing, was arrested from Feni’s Barahipur on Sunday.
He is now on police remand.
OC Khan said they had contacted the Feni Model Police Station for handing over Patwari for investigations.
Investigation Officer and Feni Model Police Station Inspector (Investigation) Abul Kalam Azad said they had been informed about the arrest.
The process to bring Patwari to Feni has started, he said.