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. interview to describe his meeting with the campaign adviser, George Papadopoulos.

The agents summarized their highly unusual interview and sent word to Washington on Aug. 2, 2016, two days after the investigation was opened. Their report helped provide the foundation for a case that, a year ago Thursday, became the special counsel investigation. But at the time, a small group of F.B.I. officials knew it by its code name: Crossfire Hurricane. [...]

Agents considered, then rejected, interviewing key Trump associates, which might have sped up the investigation but risked revealing the existence of the case. Top officials quickly became convinced that they would not solve the case before Election Day, which made them only more hesitant to act. When agents did take bold investigative steps, like interviewing the ambassador, they were shrouded in secrecy.

Fearful of leaks, they kept details from political appointees across the street at the Justice Department. Peter Strzok, a senior F.B.I. agent, explained in a text that Justice Department officials would find it too "tasty" to resist sharing. "I'm not worried about our side," he wrote.

Only about five Justice Department officials knew the full scope of the case, officials said, not the dozen or more who might normally be briefed on a major national security case.

The facts, had they surfaced, might have devastated the Trump campaign: Mr. Trump's future national security adviser was under investigation, as was his campaign chairman. One adviser appeared to have Russian intelligence contacts. Another was suspected of being a Russian agent himself.

In the Clinton case, Mr. Comey has said he erred on the side of transparency. But in the face of questions from Congress about the Trump campaign, the F.B.I. declined to tip its hand.