U.S. INTELLIGENCE agencies have publicly accused Russia of seeking to tilt the U.S. presidential election to Donald Trump. Now the nation needs a bipartisan and transparent investigation into how this occurred and how a repeat might be prevented. Mr. Trump should welcome such an inquiry.

It is important not to conflate the startling allegation of Russian interference with a separate series of unsupported accusations published by the website BuzzFeed on Tuesday. In his news conference Wednesday, Mr. Trump vehemently denied the BuzzFeed allegations but appeared to accept the judgment about Russian interference, albeit with caveats.

The intelligence agencies last Friday released an unclassified version of their conclusions regarding Russian interference. They provided a classified version, with more supporting evidence, to President Obama and Mr. Trump. CNN on Tuesday evening reported that U.S. intelligence officials also presented Mr. Obama and Mr. Trump with a two-page document describing rumors of Russian efforts to gain influence over Mr. Trump. BuzzFeed then published a memo that has been widely circulating in Washington with claims that Russian intelligence officials made a compromising video recording of Mr. Trump and met secretly with his representatives. Other news organizations, unable to substantiate these claims, had properly refrained from publishing them.

Mr. Trump angrily dismissed the rumors, via Twitter, as “FAKE NEWS — A TOTAL POLITICAL WITCH HUNT!” But he accepted at least in part the intelligence community’s finding of Russian interference, which intelligence officials said was effected through hacking of Democratic Party email accounts and the dissemination of phony news stories harmful to Hillary Clinton’s campaign.

“As far as hacking, I think it was Russia,” Mr. Trump said. He announced that he will ask his intelligence chiefs to deliver, within 90 days, a plan to harden U.S. agencies against cyber-intrusion. He pointed out that other actors, including China, have stolen information from the U.S. government.

Any effort to improve U.S. defenses should be welcomed, but Russia’s intrusion into the U.S. election is qualitatively different from even the most egregious cyberespionage. It is disturbing to hear Trump officials gloss over this difference, just as it was disturbing for Mr. Trump to welcome the results of the Russian intrusion. “Hacking’s bad, and it shouldn’t be done. But look at the things that were hacked, look at what was learned from that hacking,” he said. The comment resonated with Mr. Trump’s plea, at his last news conference a half-year ago, for Russia to find and publish Ms. Clinton’s missing emails.

Mr. Trump should be affronted by a foreign power’s interference in U.S. democracy — no matter the results. And he should recognize that the credibility of his denials of any Russian connections is undermined by his refusal to release tax returns and business records. All the more reason for a credible inquiry, beyond any actions by a Trump-controlled Justice Department or a Republican-led Senate Intelligence Committee used to operating in the shadows. The investigators of Russian meddling, whether a congressional select committee or an independent commission, should have bipartisan balance, full subpoena authority, no time limit and a commitment to make public as much as possible of what they find.