15 CFR § 90.7 - Where to file a challenge.

A request for a population estimate challenge must be prepared in writing by the governmental unit and filed with the Chief, Population Division, Census Bureau by sending the request via email to POP.challenge@census.gov or to a physical address that the Census Bureau will specify in the updated “Population Estimates Challenge Program Review Guide” to be posted in the census.gov website. The governmental unit must designate a contact person who can be reached by telephone or email during normal business hours should questions arise with regard to the submitted materials.