If lawmakers are not satisfied by the summary that Mr. Barr provides, they can begin the process of extracting more information from the Justice Department by requesting documents and subpoenaing information, including the Mueller report itself.

Congress can also call witnesses, including Mr. Mueller, to testify in a public hearing or a private deposition. And lawmakers can use their bully pulpits to publicly pressure Mr. Barr for more information, tying the Justice Department up in hearings and subpoena fights that last for the rest of Mr. Trump’s time in office.

Democratic leaders are already laying the groundwork to obtain as much information about the Mueller report as possible, saying that the department has established a precedent of giving Congress sensitive information.