The court will decide upon the quantum of punishment to the trio on May 23, the judge said.
Prabhunath and his two brothers were taken into custody soon after the judge held them guilty and were sent to the Hazaribag central jail.
According to the prosecution, Ashoke Singh, the erstwhile Janata Dal MLA of Masrakh in Chapra district, was killed at his residence in an attack involving use of bombs and indiscriminate firing by convicts on July 3, 1995.
The case had been registered on the basis of the statement of the slain MLA's widow Chandni Devi.
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