IT WAS the announcement that Washington has awaited for nearly two long, tweet-filled years: Attorney General William P. Barr told Congress on Friday that special counsel Robert S. Mueller III has completed his report on Russia’s involvement in the 2016 presidential election. The report is now on Mr. Barr’s desk. The question is how much of it will move from there into the hands of Congress and the public.

Though Friday’s announcement did not reveal the report’s contents, Mueller biographer Garrett M. Graff noted that there is significance in the fact that the special counsel was able to complete it relatively unhindered, and that the White House did not attempt a Nixonian purge at the Justice Department. “We shouldn’t have to celebrate something as basic as this, but it’s a sign that our system, our rule of law, and our democracy is holding — for now,” Mr. Graff said.

For now, indeed. As Mr. Barr considers what to release publicly, he must keep in mind that the Mueller inquiry is no ordinary investigation. Typically, the Justice Department is wary of revealing investigative information that did not lead to an indictment. This is the right instinct: It guards against law enforcement dragging people through the mud when prosecutors do not have enough evidence to charge them formally. But an attack on the country’s democracy — and senior officials’ response — is a national concern with unusual importance to the country’s politics and policy. Part of the point is to educate the public and reform the law to better prepare for further foreign intrusions. Mr. Mueller’s conclusions and supporting evidence must be released.

In the specific case of President Trump, it is also the Justice Department’s position that a sitting president cannot be indicted. It is, therefore, all the more important that any troubling information concerning his conduct be handed over to the body that can decide whether to punish a wayward president: Congress.

Unfortunately, Congress failed to pass a bill mandating transparency before Mr. Mueller handed his report to Mr. Barr. The House resoundingly passed a nonbinding resolution demanding “the public release of any report Special Counsel Mueller provides to the attorney general, except to the extent the public disclosure of any portion thereof is expressly prohibited by law.” But the Senate failed to act on it.

Fortunately, Mr. Barr promised during his confirmation hearings “to provide as much transparency as I can consistent with the law.” While carefully hedged, Mr. Barr’s statement communicated an intent to respect the public’s unusually strong interest in knowing the what and why of the Mueller investigation’s findings. In his letter to Congress, he reiterated Friday that “I remain committed to as much transparency as possible.” Now is the time to make good on his promise.