On Friday, the SSP confirmed to PTI that he had received such a letter from Bhola Singh. When asked what action was taken over it, he said probe orders were given.
The inspector and 20-year-old Sumit were killed when a mob attacked policemen over alleged cow slaughter on December 3.
IG Crime S K Bhagat said Friday that five more arrests have been made in connection with the inspector's death, taking the number of those arrested to nine.
The letter, which was surfaced on social media, was signed by BJP's Syana city chief Sanjay Shrotri, city vice president Kapil Tyagi, former corporator Manoj Tyagi, BB nagar mandal president Neeraj Chaudhary, BJP assembly convener Vijay Kumar Lodhi and Syana block pramuk Pushpendra Yadav.