The police raided Gangta village last night and arrested one Pintu Lal Dev, who is the brother-in-law of Jha's sister Munni Devi, Senior Superintendent of Police A K Satyarthi said.
Seth was being questioned by the Special Task Force in connection with the murder case of Brajesh Kumar and Mukesh Kumar, he said.
Satyarthi said that six suspects detained for interrogation have been let off on bond as their role in the engineers' murder case could not be established.
Santosh Jha, incarcerated in a Gaya jail, was under the scanner of investigators, the police officer said.
STF officials had yesterday inspected the spot of murder of the two engineers and recovered some used cartridges of AK-47 and country-made pistol, besides leaflets of Bihar People's Liberation, a gang being run by Santosh Jha.