(CNN) Editor's Note: When special counsel Robert Mueller was appointed to investigate Russian meddling in the US election, CNN, like most news organizations, endeavored to learn what we could about the famously secretive team's work. So, in addition to other newsgathering efforts, CNN had journalists who stood outside their offices. Journalists often call this a stakeout. Here, the CNN team lays out the backstory of their assignment and how this investment of time yielded big journalistic returns.

The first challenge for CNN's nearly 18-month stakeout of special counsel Robert Mueller was finding the man himself.

It all started on a chilly autumn morning in 2017, watching the sun rise over southwest Washington. After a day of staking out what we suspected to be Mueller's base of operations, a man with a red backpack crossed the street. Peter Carr was known as Mueller's spokesman, and he unwittingly led us to paydirt: a set of glass doors leading to a plain office building -- a classic example of the form in a part of town that was once an industrial hub but is now full of concrete and glass office buildings.

Over the next year, we glued ourselves to the various entrances to try to catch a glimpse of the comings and goings of the major and minor players involved in the investigation. Eventually, we discovered when Mueller zipped into work in his signature silver Subaru, and identified his team of prosecutors, FBI agents and support staff who also drove or walked into the special counsel's office.

There were garages, loading docks, front doors -- all part of the office complex. But we couldn't be at the garage and the front door at the same time to catch everything. Often, it was a gamble about where to stand and watch and risk missing something important.

Spotting Mueller beyond his daily entrance and exit in his car was difficult. And then, about a year after CNN started staking out Mueller's office regularly, out of the corner of our eyes, through the lobby window, a tall, silver-haired man came out from one hallway. Two seconds later, he disappeared into another hallway.

We ran to the side to the garage to see if we could catch the man leaving.

Success. There he was -- the elusive Robert Mueller, spotted not just coming and leaving through the garage but in the lobby.

So the coffee shop became "our spot."

Not just to see Mueller. If we were perched at the right seat, we could partially see into the building's lobby and catch glimpses of people coming and going. Not to mention the special counsel team coming in and out of the cafe themselves given its proximity and convenience.

It was a key moment for the four of us -- two different pairs of CNN reporters over the course of the 22-month investigation -- who took shifts in the daily routine of standing outside the garage and office building and sitting inside the coffee shop, observing the movements of various prosecutors and FBI personnel, marking the times and sending dispatches back to CNN's Washington bureau to help inform, and in some cases, advance the broader story.

A passerby at one point jokingly dubbed the CNN team 'TMZ', after the site that makes ample use of paparazzi. But the point wasn't to ambush people by snapping pictures. The point was to help the public know what its government was doing.

Time and time again, CNN was able to penetrate the wall of secrecy surrounding Mueller's team thanks to key observations and break news about what the special counsel was up to, from former Trump campaign chairman Paul Manafort's cooperation sessions to the secret indictment of Roger Stone

The work was sometimes thrilling, often monotonous and always continued no matter rain, sleet or snow. Countless hours were spent watching, and by extension, listening -- to music, podcasts, Russian talk radio for one journalist, or Trump personal attorney Jay Sekulow's daily radio show in hopes of hearing something Mueller-related -- to pass the time.

Even though the stakeouts could be exhausting, there were moments when the full weight of the responsibility would hit us. Often we were the only journalistic eyes and ears on the ground at Mueller's office -- not just for all of CNN, but all of the world. It was history.

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The daily plan

Our mornings followed a pretty standard routine: one of us would arrive around 6:50 a.m., set up our camera by the garage, and look for Mueller and his team to arrive. Often at least one prosecutor or FBI agent beat Mueller into the office. Most came by car, but some walked in from a nearby Metro station. One frequently rode a motorized scooter.

By mid-morning, we would move to the front entrance of the building, where a courtyard separated us from visitors, defense counsel and possible witnesses. We took hundreds of photographs of strangers we thought looked "lawyerly" entering and leaving the premises, and we were always on the lookout for clues.

At lunch, we waited for prosecutors and FBI personnel to emerge for Korean BBQ or Mexican food at the nearby food trucks, and sandwiches at the coffee shop. The daily lunch hour was our opportunity to spot anyone we had missed that morning and conduct a final headcount of who did or didn't show up to work that day.

Afternoons were generally slower -- though not always. Finally, by 5 p.m., we would sweep the area, check for any departures and then head out.

Of course, we were not invisible -- that wasn't the aim. And while the special counsel team was famously tight-lipped, sometimes they had questions for us.

On one federal holiday, prosecutor Aaron Zebley with the special counsel's office stopped to ask us if Mueller had yet to arrive, knowing we probably had the answer. (We did -- Mueller wasn't in yet.)

But the team was guarded, respecting their boss' serious "no leak" policy. Questions about the investigation were off the table. Polite hellos and 'how are yous' were the only things exchanged. (The special counsel's office is aware of this story.)

Sometimes journalists from other organizations were there, often not. And we were right in the middle of it, even if we were just standing on the sidewalk.

Paul Manafort and the ice cream social

There was a rhythm to Mueller's office -- so changes from the ordinary helped us decipher what was happening. Like on September 13, 2018, when there was free ice cream in the plaza.

Mueller's team was approaching a deal with former Trump campaign chairman Paul Manafort, but you never would have known it from the tiki-themed ice cream event the building management threw in the courtyard. The courtyard outside of Mueller's office was full of people -- including members of the special counsel team like Carr, prosecutor Michael Dreeben, and several FBI agents -- who all came down for free scoops. Carr even returned for seconds.

Then, a big clue. At 2:40 p.m., when a paralegal who we knew to be part of the Manafort defense team went into the nearby cafe, emerging with a pizza, four sandwiches and four sodas -- just enough, we surmised, for himself and Paul Manafort's three lawyers. At the end of the day, a black Dodge Durango left the Mueller's office loading dock -- half an hour after Mueller left. (The Durango was one of many ordinary, plain vehicles the special counsel's office used to transport witnesses and members of the team.)

About a week later, the proffer sessions began in earnest. While the rest of the country was gearing up for the midterms, Manafort and his legal team were visiting the special counsel on a nearly weekly basis, adding up to a total of nine sessions. It was during many of these sessions when, we later learned, Manafort was later accused of lying to prosecutors. Every morning, we were on the lookout for what would come first: a black government car with tinted windows which we figured out was transporting Manafort from jail and into the loading dock, or Manafort's lawyers coming in on foot -- often led by lead defense attorney Kevin Downing ready with a joke.

How are negotiations going with the Mueller team? "Just having lunch, tacos around the corner," he said, holding the bag open to see, before moving on.

Why they were there following Manafort's agreement to waive having an attorney present as part of his plea deal? He motioned across the plaza: "There's great sandwiches over there."

The mystery case

Over time, we trained our eyes to recognize not only Mueller from where we sat in the coffee shop but everyone else we knew who worked at the special counsel's office, whether they were walking toward us or away from us, or with a crowd of people. (The government shutdown, which drastically reduced foot traffic in the plaza, helped confirm this as well.) At the same time, any deviation caused consternation in the CNN office. One day in December 2018, Mueller wasn't seen coming in, and an email returned half-jokingly with:"SO MUELLER IS MISSING?!"

The unofficial FBI uniform of a bright white dress shirt -- Mueller does not like his team wearing shirts with patterns -- and a dark suit helped to spot people from far away, too. We realized that when a group of people -- prosecutors, FBI agents, DOJ paralegals or administrative personnel -- headed to the garage, we could catch them leaving and send a heads up to our colleagues at US District Court, roughly five minutes away. We could run to the side to the garage, too, to see which tinted car they left in and in which direction they were headed.

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We reverse-engineered this method as well; if we saw a car drive into the special counsel's office, we could run to the front to see who was coming up from the garage and into the lobby elevators.

On December 14, 2018, a hearing at the DC court of appeals about a mysterious grand jury subpoena was to occur. The case, long suspected to be related to Mueller's investigation, was sealed and secretive -- the court had taken great precautions to keep the identity of the defense lawyers and prosecutors anonymous by clearing the entire fifth floor of any journalists.

As the press corps teamed up to stake out the courthouse, we knew that members of the special counsel's office drove out in a black DOJ car that day, and they would presumably drive the car back. We waited at the side for the car to return and ran to the front where we observed prosecutors Zainab Ahmad, Dreeben, Andrew Weissmann, Elizabeth Prelogar and one other person emerged from the garage. Our stakeout again paid off helping us fill some gaps in the story we reported later that day

From spots to scoops

Of course, Manafort wasn't the only high profile witness being ushered quietly into the special counsel's office earlier that year for hours of interviews.

In January 2018, we spotted a man with a dark overcoat and brown suitcase exiting Mueller's office, flanked by two people who appeared to be legal assistants. The trio hurried out of the front entrance and got into a dark car, but we were able to snap a quick photo from our post on the sidewalk.

We later identified that the older man was a prominent white-collar attorney, Tom Green.

This led to CNN's scoop that former Trump campaign aide Rick Gates had quietly added Green to his defense team and that Gates was nearing a plea deal with the special counsel. Gates, not known for grabbing headlines like other witnesses in Mueller's probe, turned into one of Mueller's most significant witnesses, testifying against his former boss Manafort.

One night in early February 2018, someone we hadn't seen before exited the building.

We later identified the man as veteran pollster and political strategist Tony Fabrizio, which led to CNN's report that he had been interviewed by Mueller's team . Fabrizio had previously worked on Ukrainian elections with former Trump campaign chairman Paul Manafort and went on to serve as the Trump campaign's chief pollster.

The hunch that helped CNN get Roger Stone's arrest on camera

Slight deviations from anyone's routine signaled something important, we came to believe.

Prosecutor Aaron Zelinsky, for example, frequently zipped into work on a motorized scooter before walking past us and looking down at his phone. But on one memorable Thursday in January, something was different. Zelinsky walked into work with a small suitcase, a plastic bag and his usual black messenger bag. His charcoal helmet, which he wears while usually scootering or biking into the office, was noticeably absent.

We knew Zelinsky was one of the Mueller prosecutors working on matters relating to Roger Stone, Jerome Corsi and WikiLeaks. Corsi's stepson was scheduled to testify that day in front of the grand jury, and we knew there had been speculation for months that Stone could be indicted.

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Roughly half an hour later, we spotted Zelinsky and an FBI agent leaving the special counsel's office in a tinted car we had previously identified as a car used by Mueller's team. Zelinsky and Andrew Goldstein were later spotted at court for what our court reporter believed was a convening of Mueller's grand jury. Was an indictment coming?

As it happened, both Sam and Em were at the stakeout that afternoon since one of us (we won't say who) locked the stakeout car key, along with their jacket and bag, in the car. It was a chilly day, so as we waited for help we decamped to the coffee shop to stay warm -- where we then spotted Zelinsky leaving the office in casual clothes with his suitcase and briefcase. He then walked down the street to a nearby hotel, where he hailed a cab.

Earlier in the day, we thought that perhaps the suitcase contained materials or documents, but Zelinsky's early departure in casual clothing didn't make sense. Back at the CNN office, reporters covering the investigation put this clue, along with the grand jury session on a Thursday, rather than the usual Friday, together and guessed that Stone may have been indicted.

CNN sent a producer and camera to his home in Florida that night, and sure enough, less than 12 hours later, captured the dramatic pre-dawn arrest on video. Zelinsky showed up in the Florida courthouse for the Stone appearance.

The end of the investigation

By the time Mueller's investigation wrapped up last month, we had plenty of company outside the building.

Mueller's ranks of prosecutors, FBI agents and support staff deviated from routine to avoid the gaggle of cameras stationed at the front of the building and other press in general.

"Oh, here we go again," one passerby scoffed as she walked past the gaggle of cameras and into one of the neighboring buildings.

Of course, the gathered reporters knew Mueller was not about to walk out with the report in hand and give a press conference. But the press corps was ready for something to happen.

Things had already gotten a bit odd.

On one memorable afternoon in February 2019, about a month before the report was submitted, we noticed a commotion in the lobby of Mueller's office building. A few minutes later someone in a bird costume "flew" towards the lobby entrance. Whoever it was then danced, flapped their wings, and "mooned" the press corps assembled outside. They then flapped their arms and "flew" back to the elevators of the steel and glass building.

.@FossumSamuel's best update from #CNNStakeout today:

"Someone in a parrot costume went up to the door. They 'banged' their beak against the glass then showed us their parrot butt... I assume someone inside is making fun of the press out here." pic.twitter.com/8VLk5gMNkM — Katelyn Polantz (@kpolantz) February 22, 2019

The day the report was submitted, March 22, it was likely handed to the Justice Department between 11:45 a.m. and 1:10 p.m., during which we saw Mueller's head of security drive out of the building in SCO vehicles with tinted windows.

By 4 p.m., every prosecutor we spotted entering the office that morning was gone for the day.

Scott Meisler walked out with a backpack and suitcase, turning the tables and stopping briefly to take a photo of the flock of cameras outside. Rush Atkinson, Dreeben, James Quarles, Goldstein, Prelogar, Zebley and Adam Jed had all either driven or walked out of the building. Aaron Zelinsky, the last man standing, left at 3:38 p.m.

Where was Mueller? One minute later, the head of the special counsel's security sped out of the garage and drove north in the silver Subaru with no one else inside.

The press outside the garage buzzed for the first time all day. About 10 minutes later a woman in a compact car drove out of the special counsel's office building saying, "He's coming, he's coming." Then, the silver Ford expedition drove out of the building.

Was Mueller inside? We can't say for sure. But all clues suggest he was, because this car -- a vehicle that CNN's stakeout team has previously seen used to transport witnesses and persons of interest -- was likely used to bring him to work early in the morning.

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Work done, but still showing up

When Attorney General William Barr finally released the special counsel's work to the public on Thursday, April 18, a skeleton crew of staffers working out of the office showed up. While things had clearly wound down since late March, the special counsel and members of his staff continued to work -- likely to assist with redacting the public report and handing off of the 14 investigations that stemmed from his investigation.

Only a few signs pointed to the day's wider significance: Mueller's early morning and secretive arrival through the building's loading dock, and he gaggle of cameras ready to capture even the smallest glimpse of the man responsible for the nearly 450-page report that has Washington buzzing.

At lunch time, prosecutor Aaron Zebley said that they're still there for "clean up." His colleague James Quarles, with a wide smile, added what had become a familiar joke from everyone: "And the food's so good!"

"This might be the last day you see us," Zebley quipped as he walked away.

Friday morning, Robert Mueller was back at work by 7:45 a.m.