Seven persons were on Saturday arrested including the alleged mastermind behind the attack on villagers in Khagaria in which 16 people were killed.
"We have arrested seven persons, including the mastermind O P Shah and detained more than 12 persons for interrogation", Superintendent of Police I N Mishra said.
"It is a major breakthrough in the investigation into the case," Mishra said.
Regarding the involvement of Naxalites in the killing, Mishra said it was still being probed whether the carnage was "a handiwork of CPI (Maoist) ultras."
Sixteen people, including five teenagers, belonging to backward classes were dragged out of their camps and gunned down by at Amosi Bharen late on Thursday night in Khagaria district.
The Maoists had carried out the attack with the intention of grabbing land belonging to the backward classes, ADG (HQ) Neelmani had said. The victims, who belonged to Koeri and Kurmi castes of Amdaicharua village, had put up camps there.
Ten persons were detained at Alauli police station in the district for interrogation yesterday.