The person, identified as Rajiv Bujarbaruah, was picked up from Sankachi village on the suspicion of allegedly having an affair with the engineer's wife.
The bloodstained body of the engineer, Sujan Dutta, was recovered on January three from his official quarters at North Guwahati town near here.
Dutta hailed from Orissa but had been staying in Assam for many years.
Dutta's body bore injury marks suspected to have been made with sharp weapons.
Two daggers were recovered from the spot.
Bujarbaruah alleged during interrogation that he had been asked to carry out the killing at the insistence of Dutta's wife Dulu Mahanta, the police said.
