Washington, D.C.: Within hours of opening an investigation into the Trump campaign’s ties to Russia in the summer of 2016, the FBI 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 was to meet the Australian ambassador, who had evidence that one of Donald 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 FBI interview to describe his meeting with the campaign adviser, George Papadopoulos.

The agents summarised 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 FBI officials knew it by its code name: Crossfire Hurricane.

The name, a reference to the Rolling Stones lyric “I was born in a crossfire hurricane,” was an apt prediction of a political storm that continues to tear shingles off the bureau. Days after they closed their investigation into Hillary Clinton’s use of a private email server, agents began scrutinising the campaign of her Republican rival. The two cases have become inextricably linked in one of the most consequential periods in the history of the FBI.

This month, the Justice Department inspector general is expected to release the findings of its lengthy review of the FBI’s conduct in the Clinton case. The results are certain to renew debate over decisions by the FBI director at the time, James Comey, to publicly chastise Clinton in a news conference, and then announce the reopening of the investigation days before Election Day. Clinton has said those actions buried her presidential hopes.

Those decisions stand in contrast to the FBI’s handling of Crossfire Hurricane. Not only did agents in that case fall back to their typical policy of silence, but interviews with a dozen current and former government officials and a review of documents show the FBI was even more circumspect in that case than has been previously known. Many of the officials spoke on the condition of anonymity because they were not authorised to discuss the investigation publicly.

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 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.

The facts, had they surfaced, might have devastated the Trump campaign: 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, Comey has said he erred on the side of transparency. But in the face of questions from Congress about the Trump campaign, the FBI declined to tip its hand. And when The New York Times tried to assess the state of the investigation in October 2016, law enforcement officials cautioned against drawing any conclusions, resulting in a story that significantly played down the case.

Comey has said it is unfair to compare the Clinton case, which was winding down in the summer of 2016, with the Russia case, which was in its earliest stages. He said he did not make political considerations about who would benefit from each decision.

But underpinning both cases was one political calculation: that Clinton would win and Trump would lose. Agents feared being seen as withholding information or going too easy on her. And they worried that any overt actions against Trump’s campaign would only reinforce his claims the election was being rigged against him.

Trump says he is the victim of a politicised FBI. He says senior agents tried to rig the election by declining to prosecute Clinton, then drummed up the Russia investigation to undermine his presidency. He has declared that a deeply rooted cabal — including his own appointees — is working against him.

That argument is the heart of Trump’s grievances with the federal investigation. In the face of bipartisan support for the special counsel, Robert Mueller, Trump and his allies have made a priority of questioning how the investigation was conducted in late 2016 and trying to discredit it.

“It’s a witch hunt,” Trump said last month on Fox News. “And they know that, and I’ve been able to message it.”

Congressional Republicans, led by Rep. Devin Nunes of California, have begun to dig into FBI files, looking for evidence that could undermine the investigation. Much remains unknown and classified. But those who saw the investigation up close, and many of those who have reviewed case files in the past year, say that far from gunning for Trump, the FBI could actually have done more in the final months of 2016 to scrutinise his campaign’s Russia ties.

Crossfire Hurricane spawned a case that has brought charges against former Trump campaign officials and more than a dozen Russians. But in the final months of 2016, agents faced great uncertainty — about the facts, and how to respond.

Crossfire Hurricane began exactly 100 days before the presidential election. The mood in early meetings was anxious, former officials recalled. Agents had just closed the Clinton investigation, and they braced for months of Republican-led hearings over why she was not charged. Crossfire Hurricane was built around the same core of agents and analysts who had investigated Clinton. None was eager to re-enter presidential politics, former officials said.

The question they confronted still persists: Was anyone in the Trump campaign tied to Russian efforts to undermine the election?

The FBI investigated four unidentified Trump campaign aides in those early months, congressional investigators revealed in February. The four men were Michael Flynn, Paul Manafort, Carter Page and Papadopoulos, current and former officials said. Each was scrutinised because of his obvious or suspected Russian ties.

Flynn, a top adviser, was paid $45,000 by the Russian government’s media arm for a 2015 speech and dined at the arm of the Russian president, Vladimir Putin. Manafort, the campaign chairman, had lobbied for pro-Russia interests in Ukraine and worked with an associate who has been identified as having connections to Russian intelligence.

Page, a foreign policy adviser, was well known to the FBI. He had previously been recruited by Russian spies and was suspected of meeting one in Moscow during the campaign.

Lastly, there was Papadopoulos, the young and inexperienced campaign aide whose wine-fuelled conversation with the Australian ambassador set off the investigation.

Trump was not under investigation, but his actions perplexed the agents. Days after the stolen Democratic emails became public, he called on Russia to uncover more. Then news broke that Trump’s campaign had pushed to change the Republican platform’s stance on Ukraine in ways favourable to Russia.

The FBI bureaucracy did agents no favours. In July, a retired British spy named Christopher Steele approached a friend in the FBI overseas and provided reports linking Trump campaign officials to Russia. But the documents meandered around the FBI organisational chart, former officials said. Only in mid-September, congressional investigators say, did the records reach the Crossfire Hurricane team.

Steele was gathering information about Trump as a private investigator for Fusion GPS, a firm paid by Democrats. But he was also considered highly credible, having helped agents unravel complicated cases.

In October, agents flew to Europe to interview him. But Steele had become frustrated by the FBI’s slow response. He began sharing his findings in September and October with journalists at The New York Times, The Washington Post, The New Yorker and elsewhere, according to congressional testimony.

So as agents tried to corroborate Steele’s information, reporters began calling the bureau, asking about his findings. If the FBI was working against Trump, as he asserts, this was an opportunity to push embarrassing information into the news media shortly before the election.

That did not happen. Most news organisations did not publish Steele’s reports or reveal the FBI’s interest in them until after Election Day.