In a statement on Sunday, Donald Trump Jr. said he had met with the Russian lawyer at the request of an acquaintance. “After pleasantries were exchanged,” he said, “the woman stated that she had information that individuals connected to Russia were funding the Democratic National Committee and supporting Ms. Clinton. Her statements were vague, ambiguous and made no sense. No details or supporting information was provided or even offered. It quickly became clear that she had no meaningful information.”

He said she then turned the conversation to adoption of Russian children and the Magnitsky Act, an American law that blacklists suspected Russian human rights abusers. The law so enraged President Vladimir V. Putin of Russia that he retaliated by halting American adoptions of Russian children.

“It became clear to me that this was the true agenda all along and that the claims of potentially helpful information were a pretext for the meeting,” Mr. Trump said.