The March 2016 meeting was followed by a breakfast the next month at a London hotel during which Mr. Mifsud revealed to Mr. Papadopoulos that the Russians had “dirt” on Hillary Clinton in the form of “thousands of emails.” That was months before the theft of a trove of emails from the Democratic National Committee by Russian-sponsored hackers became public.

Mr. Mueller’s investigators are seeking to determine who — if anyone — in the Trump campaign Mr. Papadopoulos told about the stolen emails. Although there is no evidence that Mr. Papadopoulos emailed that information to the campaign, Mr. Papadopoulos was in regular contact that spring with top campaign officials, including Stephen Miller, now a senior adviser to President Trump, according to interviews and campaign documents reviewed by The New York Times.

The revelations about Mr. Papadopoulos’s activities are part of a series of disclosures in the past two weeks about communications between Trump campaign advisers and Russian officials or self-described intermediaries for the Russian government. Taken together, they show not only that the contacts were more extensive than previously known, but also that senior campaign officials were aware of them.

Last week, Carter Page, another former foreign policy adviser to the campaign, acknowledged to the House Intelligence Committee that he also had a private conversation with a Russian deputy prime minister on a trip to Moscow in July 2016. Mr. Page, who had previously denied meeting any Russian officials during the trip, said that he had informed at least four campaign officials about his trip beforehand and notified the campaign afterward that the Russian minister had pledged “strong support for Mr. Trump.”

Publicly, Mr. Trump and former campaign officials have tried to distance themselves from Mr. Papadopoulos. Although he once praised him as an “excellent guy,” Mr. Trump posted on Twitter that “few people knew the young, low level volunteer named George, who has already proven to be a liar.” Sarah Huckabee Sanders, the White House spokeswoman, said his involvement in the campaign was “extremely limited.”