(CNN) Josh Campbell is a CNN law enforcement analyst and former FBI supervisory special agent. His bureau career assignments included serving as special assistant to former FBI director James Comey. The following is an excerpt from his new book, Crossfire Hurricane: Inside Donald Trump's War on the FBI , Copyright © 2019. Published by Algonquin.

As the door of the Gulfstream sprang open, a frigid blast of winter air filled the cabin of the jet. The hissing of its engines soon spooled down to a faded hum, and the security agents on board began making their final preparations for our arrival. They checked weapons, tested radio communications, and ran through the schedule one last time to ensure that every movement was scripted, down to the smallest detail. During this flurry of activity, our main passenger sat calmly in front of me, nodding in rhythm to a John Legend song playing over his wireless headphones.

Two dark SUVs, flanked by police cruisers with flashing lights, pulled up to the stairs of the aircraft, and the lead security agent gave me the thumbs-up. It was only when I hit the doorway that I realized we had pulled directly alongside a Boeing 757 emblazoned with the word TRUMP on the fuselage. I had seen this impressive machine before, during trips that brought me through New York's LaGuardia Airport, and on one occasion I'd even caught a glimpse of Donald Trump himself, barreling out of a Chevrolet Suburban driven by Secret Service agents, his phone pressed to his ear as he climbed the jet's stairs, no doubt on his way to yet another campaign stop as he attempted to overcome long odds in a vitriolic election cycle. But now, as I stood there staring at a gigantic airplane with the Trump name painted in gold, I couldn't help but marvel at the showmanship and branding skills of the man who had just been elected leader of the free world. For Comey, it was now time to meet the new boss.

It was January 6, 2017, two weeks before Donald J. Trump would take the oath of office and assume the role of commander in chief, responsible for protecting the United States against all enemies, foreign and domestic. Comey had flown to New York to join his counterparts in the national security community to brief the president-elect and his transition team on their findings on actions the Kremlin had taken to interfere in the 2016 US presidential election.

Whether countering terrorists seeking to kill innocent Americans, identifying cybercriminals probing our critical national infrastructure, or uncovering foreign spies working to undermine our sacred electoral process, the mission of the men and women of the US intelligence community is a deadly serious one. The four men who traveled to New York that morning had spent nearly their entire adult lives working to protect the United States from foreign adversaries. They would now channel their expertise into a briefing intended to equip the incoming chief executive with the knowledge and tools necessary to counter an ongoing—and serious—threat to the nation.

But the director of the Federal Bureau of Investigation had one additional duty that day, something the chiefs of the Central Intelligence Agency, the National Security Agency, and the Office of the Director of National Intelligence would get a pass on. For months, a series of memos had been privately circulating among members of the media and across government that contained unverified but explosive charges against then candidate Trump. As the world now knows, Christopher Steele , a former officer with the United Kingdom's Secret Intelligence Service (MI6), had been contracted by a private investigative firm to look into Trump's background. Steele, a veteran operative, drafted a series of memos describing information about compromising material that Russian intelligence services had purportedly obtained on Trump. In addition to outlining allegations of illegal business practices that might result in Russia having leverage over Trump, the "Steele dossier," as it would become known, also included tawdry alleged details of Trump's sexual proclivities and illicit acts conducted while in Moscow. At one point, Steele thought the information so potentially damning that he approached the FBI and provided it with his reporting.

In fact, Steele was not the only one concerned that Trump might be in a compromising position with the Russians. Two powerful Republican senators had already separately approached the FBI director expressing their dismay at the revelations the former British spy had possibly unearthed. In November 2016, when Comey was speaking with legislators on Capitol Hill, Senate Intelligence Committee Chair Richard Burr (R-NC) pulled Comey aside to warn him of something very troubling. Their conversation had gone unreported until now.

"There is some material circulating," Burr whispered cryptically. "It has some disturbing things in it. I just want to make sure you're tracking."

"We are," Comey said.

"I don't need to know any more about it," Burr said, expressing his respect for the FBI's independence in addressing possible counterintelligence threats. "I just felt like I needed to make sure you were aware."

Then, in December 2016, Senator John McCain called our office and indicated that he needed to come see Comey. He brought with him a single envelope, the contents of which were a mystery to us as the senior statesman slowly walked past me and down the long walkway into Comey's office.

"He had returned from the annual Halifax defense conference in Nova Scotia," Comey later told me, "and someone he knew had given him Christopher Steele's material. He had read it enough to realize he needed to give it to me."

"I don't know what to make of this," McCain had told him, "but I know enough to know you should have it. You don't need to talk to me about it ever again."

"Thank you," Comey responded. He did not acknowledge that the FBI already had the same material.

The four intelligence chiefs decided Comey should brief the incoming president on the salacious material one-on-one, both because the FBI had originally received the information and because Comey was the only one in the group who was guaranteed to remain on the job when the new administration came in. (Unlike the heads of the other intelligence agencies, the director of the FBI serves a statutorily mandated ten-year term, and Comey was in year three.) Conscious of the personal embarrassment this sensitive brief might cause the president-elect, the FBI director opted to discuss it with Trump separately at the end of the larger briefing on Russian interference.

As our two dark Suburbans neared Trump Tower, we faced a predicament: we were early. Director of National Intelligence James Clapper , CIA director John Brennan, and NSA director Michael Rogers had all flown into Newark Liberty International Airport and were still fighting Midtown Manhattan traffic, even with marked units from the NYPD clearing their path. There was no way Comey wanted to arrive ahead of the others and break the united front the agency heads had agreed upon, and he also didn't want to find himself sitting in some reception room—with Trump transition staffers engaging him in chitchat—something he loathed to his core.

I had learned this about him early on in my tenure as his special assistant, when Comey was set to join an elected politician at an event and I'd assumed that I should carve out time for the two men to meet privately beforehand to catch up. "You assumed wrong," Comey had said dryly, taking a blue felt-tip pen out of his jacket pocket and crossing that part off the draft schedule before him in an overly dramatic fashion. It wasn't that he disliked the person; he just had little patience for politics. "I also see you've seated the two of us together," he'd said, striking through the seating chart with another stroke of the pen. "You sit next to him!"

On our drive to Trump Tower, I asked the agent seated up front to slow our roll, and he eventually found an open space a few blocks away where we could pull over and wait for the others to catch up.

"Do you have the laptop?" Comey inquired, the third time he had asked me that day.

"Check," I replied, pointing to the vehicle behind us, where an FBI communications specialist sat with a secure bag containing a laptop computer that was certified for the transmission of information classified as Top Secret.

Although the FBI director travels with an array of specialized equipment that keeps him connected 24/7 to the White House and the rest of the nation's command authority, this was the first time in my nine months working directly for him that Comey had ever asked me to make a laptop available to him immediately following a meeting. This may sound like a menial task, but Comey wanted me to understand that it was a vitally important one. Aware of the unprecedented nature of an FBI director confronting a newly elected president with explosive material about his personal life, coupled with the fact that the president's campaign was secretly under investigation for its possible ties to Russia, Comey wanted to make certain that he fully documented the interaction in writing. He would later tell me he knew it was possible the president-elect might one day lie about the exchange if it ever came to light.

It would be the first of many meetings he would feel the need to memorialize.

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"They're two minutes out," the lead security agent said over his shoulder, whispering a series of instructions to his team into the microphone clipped to the shirtsleeve at his wrist. He advised us that our vehicles and the three others carrying the national intelligence chiefs were about to converge into one long motorcade and, in a preorchestrated fashion, make the short drive through the concentric rings of Secret Service protective checkpoints. As a sea of tourists, spectators, and protesters watched from behind barricades erected along the sidewalk, the armored battlewagons moved in unison along Madison Avenue, turned left onto closed-off East Fifty-Sixth Street, and pulled up to a side entrance of Trump Tower.

Before stepping out, I handed Comey his black leather binder, which on that day carried the document that would eventually play an instrumental role in his firing.

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In the lobby, Secret Service agents held two elevators that would take the delegation part of the way up along the residential side of the building. There they crossed over to the nonresidential part and took another set of elevators to meet the Trump team. The group then entered a small, basic conference room. The only incongruous item was a giant, heavy gold curtain that had been draped along the glass wall facing the hallway. Puzzled by drapery that seemed out of place for such a drab setting, the officials were told by the Secret Service that blocking the window would permit the room to be certified for the discussion of classified information. They milled about waiting for the transition team, and were soon joined by President-Elect Donald Trump; Vice President Elect Mike Pence; incoming chief of staff Reince Priebus; National Security Adviser Designate Michael Flynn; Flynn's deputy, K. T. McFarland; then congressman Michael Pompeo; and a CIA briefer.

Director of National Intelligence James Clapper kicked off the meeting, walking the incoming administration through the same intelligence community assessment on Russian interference that President Barack Obama and his team had been briefed on the day before. It was also the same briefing the intelligence chiefs had provided that morning to the "Gang of Eight"—a bipartisan group of leaders and intelligence committee chairs and ranking members from both houses of Congress—before we had departed Washington. As Comey told me, and as Clapper has since said publicly, what struck the intelligence chiefs that day was how focused the Trump team was on demanding to know whether any votes had been manipulated—which might thereby delegitimize Trump's victory—rather than on demonstrating concern over the news that the Russians had brazenly attempted to subvert a US election.

At the end of this discussion, Clapper spoke up and told Trump there was one last piece of information they would like to present, but that Comey would prefer to do it alone. Priebus asked Trump if he would like him to remain behind, and Trump dismissively waved him off.

After the large group filed out, Comey began his prepared remarks. James Comey is not someone who typically walks into a setting having rehearsed in advance his precise wording, but this was no ordinary occasion. When Comey got to the tawdry details contained in the dossier, Trump became defensive, cutting him off and denying the allegations. "Do I look like the kind of guy who needs prostitutes?" Trump asked. He then went on to recount, unprompted, a number of allegations against him by various women, which he claimed were all false. Comey indicated that the intelligence community was aware that the claims in the dossier were unsubstantiated, but that he nevertheless wanted Trump to be aware the information was circulating through government and media circles. Trump thanked Comey for the information, signaling the end of the short one-on-one meeting.

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While Comey was still in with President-Elect Trump, it occurred to me that we might not have the luxury of making a clandestine escape. In the weeks since the election, media outlets had set up camp near the elevator bank on the first floor of Trump Tower in an attempt to catch glimpses of the dignitaries paying visits to the transition team, frequently being treated to impromptu press availabilities with Trump and whatever luminary he was meeting that day. I realized that we had not actually inquired of Trump's staff whether he would expect the intelligence officials to join him to address the media. Would the president-elect, with Comey solo at his side, try to preempt the classified document's leaking by taking it head on and describing it to members of the press gathered on the first floor? A president is permitted to declassify anything he wants, but what about a president-elect? Would Trump be violating the law if he disclosed secret information? It was a very real possibility that Trump would then hand the floor over to Comey to describe what he had just been briefed on. Although usually calm, I was suddenly nearing panic.

I grabbed one of the Trump staff members nearby and asked whether the president-elect was planning a press availability after the meeting. He shrugged, barely looking up before returning to his phone. Assuming the Secret Service would be aware of any scripted moments for the day, I walked over to three agents who were huddled over what looked like a printed schedule.

"Excuse me," I interrupted, identifying myself as a special agent on the FBI director's staff.

"Do you know if the president-elect is planning to hold a press conference after this meeting?"

"Not that I'm aware," one of the agents replied.

"Is it possible that may change?" I asked, attempting to gauge the likelihood of Trump running an audible.

The three agents looked at each other and erupted in laughter. Seeing my quizzical look, one of the agents said, "I'm sorry, man. This guy's schedule changes every two minutes. But, yes, as of this second, we are not expecting him to do any press conferences today."

Once safely back in the car, I immediately handed Comey the secure laptop. He didn't say a thing—the first time we had ever climbed into a vehicle without exchanging words—but began typing. He paused every so often to stare out the window as we navigated the city, and then went back to his writing. After about twenty minutes, and following a thorough proofread, he handed me the computer, pointing to the place on the screen where I should begin reading. I was now learning Comey's version of a meeting that would spell the beginning of the end of his career, and one that would mark the start of a veritable hurricane—a torrent of attacks on the rule of law that would risk threatening the viability of our national institutions of justice.

"One of the most bizarre meetings of my life," James Comey said woefully before resuming his gaze out the window at the passing cityscape.