The letters, sent to the woman during the past one month, threatened her to give up dealership or face dire consequences.
"The woman had not complained to the police about the letters," Ganjam SP Ashish Kumar Singh said.
No address of the senders, claimed to be supporters of a sarpanch, was found in those letters seized yesterday, police sources said.
"The letters are supporting documents for us. These show a rift between the sarpanch and the deceased over the dealership for the past few months. This might be the cause of the murder," a police officer sources said.
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Das (30) was stabbed while she was returning alone from her sister's place to her house at Rambha on Monday evening. The attackers poured petrol on her and set her on fire. She died in a hospital in Visakhapatnam the next day.
Based on her dying declaration, police have arrested four persons, including the sarpanch Manas Ranjan Nahak.
"We have formed a squad to nab the fifth accused, who is on the run," the SP said.