Atlanta-area District Attorney Fani Willis will announce “charging decisions” this summer arising from her election-interference investigation into former President Donald Trump and his allies, according to a letter she sent to local law-enforcement agencies warning of the potential for violent reactions to that key milestone in her two-year probe. In the letter addressed to Fulton County Sheriff Patrick Labat, Ms. Willis said she would be announcing charging decisions relating to the investigation sometime between July 11 and Sept. 1. Ms. Willis said she wanted to give law-enforcement agencies sufficient time to prepare for the “significant public reaction” her announcement could...