In all, donors contributed $2.3 million in the last month of the 2015 fiscal year, boosting the annual contributions for the year to $3.9 million, according to just-released data from the Bureau of the Fiscal Service. Treasury officials say September’s haul was helped by a single $2.2 million payment, but would not provide any details on the source of the contribution.

A little-known account called the Public Debt Reduction Fund receives tax-deductible contributions from the public to pay down the national debt. Contributions to the account are sometimes as little as just a few cents, and come from online donations, contributions made through the Internal Revenue Service and hand-written checks to the Bureau of Public Debt in Parkersburg, W.Va. The biggest months for contributions to the fund are usually April and December.