AP offers this report, which begins: “The government’s top campaign finance regulator says John McCain can’t drop out of the primary election’s public financing system until he answers questions about a loan he obtained to kickstart his once faltering presidential campaign.” You can find the FEC’s letter to Sen. McCain here (see also McCain’s letter to FEC and McCain loan documents).
From the FEC letter to McCain:

[W]e consider your letter as a request that the Commission withdraw its previous certifications [of you as a candidate entitled to matching funds in the presidential primary season]. Just as 2 U.S.C. s 437c(c) requires an affirmative vote of four Commissioners to make these certifications, it requires an affirmative vote of four Commissioners to withdraw them. Therefore the Commission will consider your request at such time as it has a quorum.
We note that in your letter, you state that neither you nor your committee has pledged certification of Matching Payment funds as security for private financing. In preparation for Commission consideration of your request upon establishment of a quorum, we invite you to expand on the rationale for that conclusion, including but not limited to addressing the following provisions of the loan agreement executed between John McCain 2008, Inc. and Fidelity and Trust Bank of Bethesda….