Washington (CNN) Early each morning, special counsel Robert Mueller arrives wearing his signature uniform: a crisp white dress shirt, tie, slacks and dark baseball cap. Occasionally he accessorizes with a dark vest or sunglasses, depending on the weather.

A simple routine for the head of a very complicated investigation -- one that is heavily scrutinized but nearly impossible to read.

As the Mueller probe hits its one-year anniversary, the special counsel's team has brought charges against 22 people and companies, notched five guilty pleas and seen one person sentenced. While a number of those charges were related to Russia's interference in the 2016 presidential election, so far none of them has extended to potential collusion between the Russian government and Trump associates.

But under President Donald Trump's Twitter glare, Mueller's team has pressed forward with their work. Their operation reflects the man who leads it: no frills, no drama, no leaks. And that has left Washington and the nation guessing at where their pile of evidence might lead them -- and when they will arrive there.

For months, the word among lawyers representing witnesses and subjects in Mueller's investigation was that the Russia probe appeared to be nearing its end, perhaps by early this summer.

More recently, some of those prognostications are changing as Mueller's team continues to bring back witnesses for follow-up interviews, and in some cases for testimony before a Washington, DC, grand jury.

Meet the Mueller team Special counsel Robert Mueller assembled a team of at least 17 lawyers for his investigation of Russian interference in the 2016 election and potential collusion between the Trump campaign and Russia.

The activity elsewhere comes as lawyers representing Trump are in a standoff with Mueller's team over whether the President will cooperate with a voluntary interview. The negotiations have dragged on, in part, because the President's struggled to keep lawyers and hire new ones, giving way to a new legal team that is still getting up to speed.

"The chances now are 50-50," Rudy Giuliani, one of the recent additions to the President's legal team, told CNN of the odds of Trump sitting for an interview. "At times it's been higher, at times lower. I don't know what they're going to give us in the way of ground rules that make us comfortable."

Other members of the legal team give much smaller chances that a voluntary interview will happen, and are prepared to dare the special counsel to issue a subpoena, which could trigger a lengthy legal fight.

"We are going to try as best we can to put the message out there that it has been a year, there has been no evidence presented of collusion or obstruction, and it is about time for them to end the investigation," Giuliani said in an interview with Bloomberg.

Giuliani told CNN he believes Mueller is "sensitive" to the fact that his investigation is reaching its one-year mark, in part because Mueller mentioned the anniversary in a recent meeting.

"He doesn't want to look like one of these special counsels that hangs on forever," Giuliani told CNN, based on his conversation with Mueller and prior knowledge of how Mueller operates.

The special counsel's office declined to comment for this story.

But by white-collar criminal investigation standards, the Mueller probe is still relatively short. Cases of this complexity — the Enron investigation, for instance — routinely take several years to investigate.

Even as the President and his lawyers publicly clamor for an end to the investigation, privately his legal team appears content to wait out Mueller, knowing the longer the investigation goes on, the easier it is for Trump and his allies to dismiss the entire probe as a "witch hunt."

"Congratulations America, we are now into the second year of the greatest Witch Hunt in American History...and there is still No Collusion and No Obstruction," Trump tweeted Thursday. "The only Collusion was that done by Democrats who were unable to win an Election despite the spending of far more money!"

Congratulations America, we are now into the second year of the greatest Witch Hunt in American History...and there is still No Collusion and No Obstruction. The only Collusion was that done by Democrats who were unable to win an Election despite the spending of far more money! — Donald J. Trump (@realDonaldTrump) May 17, 2018

Trump's allies have been encouraged by recent polls showing Trump's approval rating inching up and Mueller's beginning to slide, particularly among Republicans, after months of withering attacks on the investigation by Trump and his allies. Trump has remained adamant that Mueller won't find any evidence of collusion -- or at least nothing that could personally implicate Trump, sources said.

"He has learned to do a better job of compartmentalizing," a Trump ally said about Trump's mindset around the Russia probe, although the source acknowledged the President still likes to vent about the investigation. "It's there, it's a pain in the ass, but it's not the dominant feature anymore."

Meanwhile, Trump's legal team is preparing for the possibility that Mueller may choose to issue a subpoena to interview the President -- a move that could lead to a lengthy legal battle that could wind up in front of the Supreme Court.

"We don't particularly want to have it," Giuliani said of a constitutional fight. "If we don't get the ground rules that we consider fair, we're going to have it."

As for Mueller, he has demonstrated -- under the supervision of Deputy Attorney General Rod Rosenstein -- that he's willing to use aggressive means to collect information. FBI agents, for instance, took the extraordinary step of stopping at least two Russian oligarchs after their private jets landed at US airports earlier this year to question them.

But Mueller has also signaled that he understands the value of drawing boundaries around the Russia probe.

In April, FBI agents carried out a series of raids on the New York office, home and hotel room of Michael Cohen, the President's longtime personal lawyer. They carted off thousands of documents and federal prosecutors in Manhattan said in court that it related to a criminal investigation of Cohen that was based at least partly on a referral from Mueller's office. So far, the Cohen probe appears to have less to do with Russia than with Cohen's business conduct, which includes his role as "fixer" for the President in handling matters such as a hush-money payment days before the 2016 election to an adult film actress who claims to have had an affair with Trump.

An investigation that expanded

The centerpiece of Mueller's investigation -- his original mandate -- was to investigate Russian interference in the 2016 election and any potential collusion between Trump and his associates and the Russian government. But the mandate also allowed Mueller to look into any areas that may arise from the investigation. So the probe expanded to include an offshoot investigation into whether the President has tried to obstruct justice.

Now, a year in, Mueller's areas of interest have run the gamut.

His team has been in contact with corporations including pharmaceutical giant Novartis and telecommunications conglomerate AT&T regarding their payments to Cohen. Investigators have pressed witnesses about the extent of longtime Trump adviser Roger Stone's contact with WikiLeaks founder Julian Assange. They've asked Russian oligarchs whether wealthy Russians sent cash illegally, either directly or indirectly, to Trump's presidential campaign an inauguration.

Others have been questioned about former Trump national security adviser Michael Flynn's contacts with the former Russian ambassador and Flynn's subsequent attempts to cover it up. Some have been asked about the President's decision to fire FBI director James Comey. Still others have been questioned about how a statement was crafted aboard Air Force One, which misleadingly claimed a 2016 Trump Tower meeting between senior campaign aides and a Russian attorney focused on primarily adoptions rather than collecting damaging information about Hillary Clinton.

Those who have met with Mueller's team have left stunned by what investigators already know. By the time someone is called in for an interview, investigators have already crafted meticulous timelines based on evidence, such as emails and calendars, of the events they plan to ask about.

Still, witnesses are often left with only a glimpse of where the investigation might be headed. Some lawyers have acknowledged they have no idea why their clients were asked certain questions.

What it's like to be interviewed by Mueller's team

The special counsel's team has toiled quietly and declined to hold press conferences. When Mueller brought charges against 13 Russian nationals and three Russian entities in February for allegedly meddling in the 2016 presidential election, he left it up to Rosenstein, who oversees the Russia probe, to make the public announcement.

Still, Mueller's team is well aware of the media glare it's operating under.

Prosecutors often warn lawyers and other visitors that there may be news cameras staked outside the nondescript government building in Southwest Washington where Mueller's team works. And those who have been interviewed by the special counsel's team don't stop to chat. That's how Mueller's team likes it.

Mueller's team often sets up rendezvous point with witnesses, who are then picked up in a rotating fleet of cars with tinted windows. A white sedan, a black SUV and a teal car with a dent in the side have all been used to whisk witnesses through the loading dock and into a parking garage minutes before their scheduled interviews.

Those who have been willing to walk through the front door -- in full view of the media -- have been discouraged from doing so, according to sources familiar with the setup.

"It's a very nondescript, very ugly government office," former Trump campaign adviser Michael Caputo told CNN's Anderson Cooper after his interview with Mueller's team in May. "They're all business over there. And they're not looking for any luxury."

After the covert entry, lawyers and clients are confronted with a wall of lockers where visitors are instructed to stash their cell phones and other electronics before proceeding. Most of the meeting room space is a secured Sensitive Compartmented Information Facility, or a SCIF, so cell phones won't function inside.

Witnesses are deposited in a drab waiting room where doors are secured with electronic locks and bureaucrats are buzzing in and out.

The questioning takes place in relatively tight quarters -- conference rooms large enough to fit a table, a few chairs and little else. Mismatched tables and chairs -- all in muted tones -- look like they've come from an office surplus store and then bounced through multiple government buildings before ending up wedged between Mueller's prosecutors and their latest witness. There's no refreshment station, although interviewees are offered water.

Prosecutors take the lead on questioning witnesses, with FBI agents occasionally chiming in with facts investigators have found. Mueller himself is rarely seen, sources said. When a witness wants to confer with their lawyer, investigators step out of the room.

"The main thing I noticed was it was a very pressured environment for the people working there. They were friendly but not particularly relaxed," one lawyer who has visited the office said.

Those who go to the grand jury at federal district court in Washington have a more formal experience

"It is sort of like being in a school room or a university lecture room" with "tiers of jurors" facing the witness, said one person who testified.

The three tiers of jurors represent a "mixed bag of people," the person said, describing "a non-descript group of what looked like a cross section of humanity" that lives in Washington.

After being sworn in, prosecutors come at you "methodically" with no "grandstanding," as they ask you questions or present documents for you to explain.

At the end, a simple thank you for your time is offered on behalf of the grand jury.

"You are relieved that you've done it," said the witness.

Members of Mueller's team keep habits similar to any Washington bureaucrat, albeit with a far more secretive to-do list.

Prosecutors and FBI agents mostly trickle in by car or via Metro, with the notable exception of one prosecutor who bikes to the office daily -- even when temperatures dropped below freezing.

Some are more recognizable. Andrew Weissmann, a prosecutor on the team, has a signature look of his own: a rotating selection of leather backpacks.

They venture out for lunch, popping by a Mexican food truck, nearby pizza shop or the roving popcorn truck. Also available: an employee lunchroom that smells vaguely of Hot Pockets.

While investigators carry on, their work shrouded in mystery, Americans are overwhelmingly united on at least one point: They want to know what Mueller is up to.

According to a CNN poll conducted by SSRS, 84% of Americans said Mueller should issue a public report at the end of his investigation, regardless of what he finds.