Before arranging a meeting with a Kremlin-connected Russian lawyer he believed would offer him compromising information about Hillary Clinton, Donald Trump Jr. was informed in an email that the material was part of a Russian government effort to aid his father’s candidacy, according to three people with knowledge of the email.

The email to the younger Mr. Trump was sent by Rob Goldstone, a publicist and former British tabloid reporter who helped broker the June 2016 meeting. In a statement on Sunday, Mr. Trump acknowledged that he was interested in receiving damaging information about Mrs. Clinton, but gave no indication that he thought the lawyer might have been a Kremlin proxy.

Mr. Goldstone’s message, as described to The New York Times by the three people, indicates that the Russian government was the source of the potentially damaging information. It does not elaborate on the wider effort by Moscow to help the Trump campaign.