Although the Treasury market has been the traditional vehicle for monetary policy operations in recent decades, the Federal Reserve Act provides authority for the Federal Reserve to purchase a fairly wide range of other assets - including obligations of federal agencies, certain obligations of state and local governments, foreign exchange, and sovereign debt.
Moreover, the Federal Reserve often supplies reserves through repurchase transactions in addition to outright purchases. If the Treasury market became less liquid, we could substitute longer-term RPs against eligible collateral for some outright purchases of securities.