A village organisation head was arrested on Monday while a JDU leader and two others were detained a day after an RTI activist and his friend were shot dead by unidentified assailants in Bihar's Jamui district, police said.
Meanwhile, hundreds of angry residents blocked a road, burnt tyres and shouted slogans against the state government to protest the killings, disrupting traffic on National Highway 333.
Valmiki Yadav and his friend Karu Yadav were attacked by motorcycle-borne assailants near Bichwe village on Sunday evening.
Village body head Krishna Ravidas was arrested and JDU block president Suresh Mahto and two others were detained for interrogation, said Sub Divisional Police Officer Rampukar Singh.
"Prima facie, it seems to be a pre-planned conspiracy," he said.
Mr Yadav is said to have exposed several rackets and financial irregularities in the public welfare schemes and developmental works in the district.
This is the third such crime in the last three months. A Right to Information (RTI) activist was shot dead in Vaishali district in April whereas another RTI activist was shot dead in East Champaran district last month.