In an April statement to The New York Times regarding a different matter, the F.E.C.’s chairwoman, Dara Lindenbaum, a Democrat, and its vice chairman, Sean J. Cooksey, a Republican, said: “We take this agency’s enforcement and transparency mission seriously, as do all of our colleagues. Without commenting on any specific case, Commissioners assess each enforcement matter on its merits, and we reach agreement in nearly 90 percent of them. Any claim that the Commission is ‘toothless’ or that its bipartisan structure prevents it from fulfilling its mission is misinformed.”

Even as the F.E.C. sought to respond to Mr. Santos’s actions with its usual enforcement mechanisms, those tools were not on their own able to address the extent of the brazen misconduct he has been accused of. In the months before Mr. Santos’s election, the F.E.C. sent his campaign a series of “requests for additional information” — commonplace inquiries seeking details about transactions not fully accounted for in a committee’s regular filings with the commission.

The F.E.C. does not have the power to look in bank accounts and must take campaign finance disclosure reports at face value. Unlike the Securities and Exchange Commission, it is not a regulatory agency — its primary mission, laid out in its most recent annual report, is to provide transparency and “promote compliance.”

Some questions about Mr. Santos’s campaign remain unanswered. His campaign’s filings are filled with suspicious expenditures — for example, some of the dozens of charges pegged at $199.99, cents below the federal threshold that requires receipts — and outright omissions, such as hundreds of thousands of dollars in spending that was never accounted for.

“Unfortunately, we don’t know what we don’t know,” said Paul S. Ryan, a campaign finance expert who previously worked at the Campaign Legal Center. “The bottom line here is that the only thing the F.E.C. sees, in most instances, at first glance, are the campaign finance disclosure reports,” he said. “And if someone is smart enough to simply lie about what they are using it for,” then it can go overlooked, he said.