Seven persons were on Saturday arrested in connection with the attack in Khagaria which claimed the lives of 16 villagers.
"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 Naxals in the killing, Mishra said it was still being probed whether the carnage was 'a handiwork of Communist Party of India - Maoist ultras'."
Sixteen people, including five teenagers belonging to backward classes, were dragged out of their camps and gunned down 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 victims, Additional Director General 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 on Friday.
The superintendent of police said while the matter related to a dispute over a plot of land being cultivated by the victims, the police is looking into some other angles as well. Mishra said an FIR has been lodged with the Morkahi police station in which 37 persons have been named.
The case was also registered against some unknown persons who might be involved in the conspiracy to assemble the marauders. Deputy Chief Minister S K Modi had castigated the opposition Rashtriya Janata Dal - Lok Janshakti Party front for allegedly 'making sinister attempts to foment caste tension' and said the probe would also cover 'the political conspiracy angle' to ascertain whether it was a ploy to defame the state's National Democratic Alliance government.
Chief Minister Nitish Kumar had announced an ex-gratia of Rs 1.5 lakh, including Rs 50,000 from the CM's relief fund, to dependents of each of the deceased.