The robbery took place early Sunday.
The detainees are - Gazi Arman, 24, son of local BNP leader Gazi Abu Taleb, Monir Hossain, 27, son of Chanu Miah, Khalil Hossain, 19, son of Mokbul Hossain and Akter Hossain, 25, son of Abul Hossain. They are residents of the same upazila.
Araihazar’s Khagkanda police outpost In-charge Shariful Islam said a gang of six to seven pirates targeted a goods-laden boat in the river and they beat up the workers durin looting.
“At that time, patrolling river police rushed to the spot hearing the uproar ensued there. Later, police detained four of the robbers while the others managed to fled the scene,” he also said.
Two guns, a metal rod and a speed boat were seized from their possession.
A case has been filed with Sonargaon police station in the incident, Shariful added.