A copy of the letter dated March 8 was posted on Treasury website:

"Treasury anticipates it will need to start taking certain extraordinary measures in order to temporarily prevent the United States from defaulting on its obligations"

Treasury will suspend the sale of State and Local Government Series (SLGS) securities, which count against the debt limit, as of March 15 and "until the debt limit is either raised or suspended" As it's done in the past, Treasury will use additional extraordinary measures. Mnuchin says that "honoring the full faith and credit" of U.S. outstanding debt is a critical commitment.