Taken together, the meetings show that early in the presidential race, people at the heart of Trump campaign and on its fringes were aware that Russian government officials were trying to help Mr. Trump.
The professor whom Mr. Papadopoulos met was not identified in court documents. The professor introduced Mr. Papadopoulos to a woman identified as a relative of the Russian president, Vladimir V. Putin, and to someone in the Russian Ministry of Foreign Affairs. Mr. Papadopoulos repeatedly tried to arrange a meeting between the Trump campaign and Russian government officials, court records show.
“We are all very excited by the possibility of a good relationship with Mr. Trump,” the woman, who was not identified, told Mr. Papadopoulos in an email.
Mr. Papadopoulos told the F.B.I. in January that the professor was “a nothing.” But Mr. Papadopoulos now acknowledges that he knew the professor had “substantial connections to Russian government officials.” Attempts to reach Mr. Papadopoulos on Monday were not successful.
The United States government has concluded that Russia hacked Democratic email accounts and released thousands of embarrassing messages related to Mrs. Clinton’s campaign. The emails began appearing online in the summer of 2016. The Trump campaign has repeatedly denied any inside knowledge about that.
Mr. Papadopoulos was one of a small group of foreign policy advisers that Mr. Trump announced in March 2016. Another of the advisers, Carter Page, has met with the F.B.I. about his own meetings with Russians.
The plea was unsealed the same day Paul Manafort, the former Trump campaign chairman, and his longtime associate Rick Gates were indicted on charges of money laundering and conspiracy.