Why did Mattis never respond to Grassley’s letter, requesting the release of exculpatory information re: General Flynn?

A question: Why did Mattis never respond to Grassley's letter, requesting the release of exculpatory information re: General Flynn? "the declassification would be in the public interest, and is in the interest of fairness to Lt. General Flynn." https://t.co/2aSlnwJqgr