The investigation may last for at least another year, with Mueller's office continuing to ask for new documents linked to the campaign, the newspaper reported on Monday, citing people familiar with the ongoing probe.
Some of the members of Mueller's team reportedly told others that they were expecting to continue their work throughout much of the next year.
Adam Schiff, a senior member of the Democratic Party of the House Intelligence Committee, said on Friday he was worried that the Republican Party might try to shut down the House investigation into the Trump campaign at the end of December and may put pressure on Mueller's investigation as well.
The US Congress is currently investigating Russia's alleged meddling in last year's election, while another probe, dubbed a "witch hunt" by President Trump, is conducted by the FBI and the Department of Justice led by Special Counsel Robert Mueller.
A number of senior Russian officials, including President Vladimir Putin, have repeatedly denied the "groundless" allegations of Moscow's attempts to influence the 2016 presidential election, pointing out that no evidence to support the claims was provided.