The body of 65-year-old Rajendra Mishra, who was a member of the district committee, was found in front of the Rupadeeha Police Station on the Indo-Nepal border yesterday, they said.
Mishra's brother has lodged an FIR against an advocate, his brother and a woman accusing them of poisoning the BJP leader.
In the postmortem report, suspicion of poison had been expressed, police said.
Mishra had an old enmity with the advocate and was also a witness against the latter and others in a murder case, they said.
Police said the woman had earlier lodged a case of sexual molestation against Mishra.
Meanwhile, BJP district president Gulab Chandra Shukla and city unit in-charge Jai Prakash Awasthi have threatened to launch an agitation if the accused are not arrested in two days.
Pics: Prabhas, Tamannaah Bhatia at 'Baahubali 2' first look launch at MAMI
'Ae Dil Hai Mushkil' photos: Aishwarya, Anushka, Karan get candid at MAMI
Photos: Huma Qureshi, Neha Dhupia and other celebs at MAMI
Photos: Amy Jackson, Daisy Shah at Mumbai airport
Photos: Sridevi, Irrfan and other celebs at 'Mughal-e-Azam' screening