A team of police from Bongaigaon district arrested the KLO militant Putul Ray from Golakganj in Dhubri district.
Pathak was kidnapped along with his colleague Faizur Islam on May 2 by four unidentified armed men at Kakragaon while returning to Guwahati from Dhubri in a car.
His body bearing bullet wounds was recovered the next day from Baitamari area in Abhayapuri.
Islam was rescued from Salkosa, the tri-junction of Kokrajhar, Bongaigaon and Dhubri districts by police on May 27 after a 25-day long search.
Islam is being interrogated by CID over his alleged involvement in the murder, the police said.