"My father has lost his life in this Hindu-Muslim dispute. Whose father is next?" Abhishek Singh said.
Inspector Subodh Singh, who had initially probed the 2015 lynching of Dadri resident Mohammed Akhlaq over rumours about beef consumption, and a 20-year-old man were killed on Monday during an attack on policemen.
The inspector's sister alleged the killing was a conspiracy by the police.
"My brother was killed in a conspiracy by the police as he was probing a cow slaughter case (the Akhlaq lynching case)," Sunita Singh told reporters on Tuesday.
"He should be given a martyr's status and a memorial should be constructed in his name in our native place," she said.
"The cow is our mother, I accept that. My brother has given his life for her. The CM used to chant 'gau, gau, gau'. Why can't he come for cow protection?" she said in an emotional outburst against Chief Minister Yogi Adityanath.
Abhishek Singh said the last time his father called he asked about his studies and if he had eaten.