Rather than the now-infamous Trump dossier, it appears to have been the activities of George Papadopoulos that initiated the FBI's interest in the Trump campaign's dealings with Russia, according to the New York Times.
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.
Exactly how much Mr. Papadopoulos said that night at the Kensington Wine Rooms with the Australian, Alexander Downer, is unclear. But two months later, when leaked Democratic emails began appearing online, Australian officials passed the information about Mr. Papadopoulos to their American counterparts, according to four current and former American and foreign officials with direct knowledge of the Australians’ role.
Papadopoulos has since pleaded guilty to lying to the FBI and is a cooperating witness.