EVERY AMERICAN should be relieved that electoral officials across the country have determined that the level of voter fraud was negligible in the 137.7 million ballots cast in November. President-elect Donald Trump’s claim of millions of fraudulent votes is simply not true. But every American should be deeply concerned about evidence that Russia attempted to use computer hacking, fake news stories and perhaps other methods to undermine the integrity of the U.S. campaign and, specifically, to defeat Hillary Clinton. Congress should establish a bipartisan, select committee to investigate.

The story so far, according to a consensus of the intelligence community, is that hackers associated with Russia penetrated the computer systems of the Democratic National Committee, leaked embarrassing emails during the campaign and did the same with Clinton campaign chairman John Podesta’s email account, all intended to create uncertainty, undermine Ms. Clinton and elect Mr. Trump. At the same time, Russian propaganda outlets and some organizations in the shadowy world of fake news also chipped in to erode Ms. Clinton’s candidacy.

President Obama has ordered an investigation to be completed before he leaves office, a necessary step. He should release as much information as possible. But it can’t end there. Sen. John McCain (R-Ariz.) has wisely led a bipartisan effort by four senators to launch a select committee investigation with a special focus on cybersecurity. Senate Majority Leader Mitch McConnell (R-Ky.) has expressed reluctance to set up a special panel, but we think it would guarantee a stronger outcome, one that would have credibility and prove effective at excavating the truth, especially if Mr. Trump continues to obfuscate and dodge.

The point is not to cast doubt on Mr. Trump’s election. No one will ever know what combination of factors, including Clinton campaign errors, Trump campaign strengths, the ham-handed FBI letter and the dissatisfied mood of the country, produced Mr. Trump’s victory. But every American should be concerned about foreign intervention in U.S. democratic processes. A select committee could examine all cases of cyber-intrusion, not only against the campaigns but also aimed at voter registration databases. In a calm and rational way, the country needs to learn all it can about attempts to contaminate or corrupt the electoral process.

Mr. Trump’s comments on this matter only underline the need for an inquiry. In July, Mr. Trump urged Russia to meddle in Ms. Clinton’s emails. He has repeatedly dismissed claims that Russia was attempting to help his campaign. He has sharply criticized the U.S. intelligence community for its findings of Russian interference. All this raises more questions. Mr. Trump would be better advised to accept an investigation and cooperate with it.

Active interference with the American election by Russian President Vladimir Putin and his security services is intolerable. The United States is an open society and must respond to this assault by exposing it to sunlight.