Within hours of opening an investigation into the Trump campaign’s ties to Russia in the summer of 2016, the F.B.I. dispatched a pair of agents to London on a mission so secretive that all but a handful of officials were kept in the dark. Their assignment, which has not been previously reported, was to meet the Australian ambassador, who had evidence that one of Donald J. Trump’s advisers knew in advance about Russian election meddling. After tense deliberations between Washington and Canberra, top Australian officials broke with diplomatic protocol and allowed the ambassador, Alexander Downer, to sit for an F.B.I....