The FIR was registered last night against Gaurav Singh, son of BJP MLA Bawan Singh from Katra Bazar, and five others, police said.
Gaurav was killed in an exchange of fire with security guard of a private hospital in Kotwali area of here on May 8.
He had gone to the nursing home for some work and had a dispute with the guard over parking of his vehicle.
The police had earlier arrested six persons including owner of the hospital Rajesh Pandey.