Markets are growing more anxious about the risk of a U.S. default if the debt ceiling isn't raised by Congress before the deadline.

Joseph Cotterill posts this chart from BofA/ML showing the interest rates on various short-term government T-Bills.

As you can see, right around the "danger zone" (around October 17, when the debt ceiling is due to be breached) investors are demanding a higher yield to hold those assets, reflecting some degree of concern and risk that the government's timely redemption of these bonds won't occur.