Add garlic, chili powder and pepper flakes; cook and stir 1 minute. Add ketchup, brown sugar, molasses, vinegar, Worcestershire sauce and mustard; heat to boiling. Reduce heat to low; simmer uncovered 10 to 15 minutes, stirring occasionally, until sauce thickens slightly. Remove from heat. Add 1/2 cup of the sauce to slow cooker; cover and refrigerate remaining sauce.