Gunmen shot dead four members of a family and critically injured another in Gaya town, police said on Friday.
The dead were identified as Chandradeo Yadav, his wife Mamoti, and their two daughters, Radhika and Anita.
The incident, which sparked off violence, took place in Delha on Thursday night, Superintendent of Police Sanjay Singh said.
As news of the incident spread in the morning, hundreds of people attacked the Delha police station and set it ablaze, destroying several official documents and furniture.
They also burnt several vehicles, including two buses and a police jeep.
The police fired several rounds in the air to disperse the crowd, Singh said, adding no casualty was reported in the firing.
Twenty-five people, including two Communist Party of India (Marxist-Leninist) activists, were taken into custody in connection with the attack on the police station.