During a night of heavy drinking at an upscale London bar in May 2016, George Papadopoulos, a young foreign policy adviser to the Trump campaign, made a startling revelation to Australia’s top diplomat in Britain: Russia had political dirt on Hillary Clinton…

The hacking and the revelation that a member of the Trump campaign may have had inside information about it were driving factors that led the F.B.I. to open an investigation in July 2016 into Russia’s attempts to disrupt the election and whether any of President Trump’s associates conspired…

The information that Mr. Papadopoulos gave to the Australians answers one of the lingering mysteries of the past year: What so alarmed American officials to provoke the F.B.I. to open a counterintelligence investigation into the Trump campaign months before the presidential election?

It was not, as Mr. Trump and other politicians have alleged, a dossier compiled by a former British spy hired by a rival campaign. Instead, it was firsthand information from one of America’s closest intelligence allies.