WASHINGTON — The still-secret report on Russian interference in the 2016 election submitted last week by the special counsel, Robert S. Mueller III, was more than 300 pages long, the Justice Department acknowledged on Thursday.

Mr. Barr wrote to Congress on Sunday offering what he called the “principal conclusions” of the report — including that Mr. Mueller had not found that the Trump campaign had taken part in a conspiracy to undermine the election. But he had notably declined to publicly disclose its length.

The total of 300-plus pages suggests that Mr. Mueller went well beyond the kind of bare-bones summary required by the Justice Department regulation governing his appointment and detailed his conclusions at length. And it raises questions about what Mr. Barr might have left out of the four dense pages he sent to Congress.

Answering those questions is likely to prove difficult for lawmakers and the public. Mr. Barr has indicated to two congressional chairmen that it will most likely take weeks to redact the report for classified and grand jury information the department deems unfit for public consumption.