Singh was being interrogated two days after Samresh Singh, a supporter of an independent candidate in Nawada parliamentary constituency in the Lok Sabha polls, was murdered in Kochgaon village, 150 km from here.
An FIR was lodged Tuesday naming the LJP candidate and his henchmen as suspects.
"Police were interrogating Singh (for his alleged involvement) in the murder," a police official in Nawada said.
Singh, convicted and sentenced to life for the murder of a farmer in 1992, has been barred by a court from contesting polls. He is out on bail after appealing his conviction. His wife Veena Devi is the LJP candidate from Nawada.