Police on Tuesday raided the divisional jail in Purnea and recovered cellular phones, SIM cards and chargers from some wards.
The raids follow a recent spurt in criminal activities in the district.
Police Superintendent Sudhanshu Kumar said policemen seized three cellular phones, eight chargers and several SIM cards.
The raids were conducted following reports that some prisoners had masterminded the killing of a businessman in the town a week ago along with the kidnapping of the schoolboy on Monday, he said.
The recovery of the SIM cards would help establish links of the prisoners with gangsters in Purnea, they said. ''We are going to transfer some of the criminals from Purnea to Bhagalpur jail,'' he said.
He added a list of such inmates was being prepared.