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It was one year ago Thursday when Robert Mueller, the former FBI director, was appointed as special counsel to take over the Justice Department's investigation into possible coordination between Russia and Donald Trump's 2016 presidential campaign. The 12 months since have yielded a flurry of criminal indictments and guilty pleas, tense court appearances, angry tweets from the president and speculation over what the ever-taciturn Mueller already knows and what he'll investigate next. Hundreds of pages of court filings, and public statements from witnesses, have to an extent pulled back the curtain on an extraordinarily secretive investigation. But much remains hidden from view. A look at the last year and what may lie ahead:

What is Mueller investigating?

There's no doubt Mueller's investigation is far-reaching, but at its core, prosecutors have remained focused on two central questions: Did the Trump campaign collude with the Kremlin to tip the 2016 presidential election in the Republican candidate's favor? And has President Trump tried to obstruct the investigation since taking office through actions including firing FBI Director James Comey and badgering Attorney General Jeff Sessions over his recusal from the Russia probe?

Who has been questioned so far?

A veritable who's who of current and former White House officials, as well as foreign businessmen and top campaign and transition staffers. Trump's son-in-law, Jared Kushner, was questioned last fall about former White House national security adviser Michael Flynn. Trump's White House counsel, Don McGahn, has been in to see Mueller's team, as have former chief of staff Reince Priebus, former communications director Hope Hicks, former chief strategist Steve Bannon, California real estate developer and longtime Trump friend Tom Barrack — and dozens of other witnesses. In a reflection of the wide-angle nature of the investigation, and its ongoing examination of foreign influence on the Trump administration, a grand jury in Washington has heard from a Lebanese-American businessman who joined a 2016 meeting at Trump Tower involving top Trump aides and the crown prince of Abu Dhabi. A Russian American lobbyist who attended a June 2016 meeting at which Trump's eldest son expected to receive damaging information about his father's opponent, Hillary Clinton, also has provided testimony.

Who has been charged?

A total of 19 people and three Russian companies have either been indicted or pleaded guilty to criminal charges. Among those charged are Trump campaign chairman Paul Manafort, who is awaiting trial in Virginia and the District of Columbia, and Flynn, who pleaded guilty in December to lying to the FBI and has been cooperating with Mueller's investigation. Other cooperators include deputy campaign chairman Rick Gates and former campaign foreign policy adviser George Papadopoulos. A Dutch lawyer who lied to the FBI is serving a 30-day prison sentence, and a California man who unwittingly sold bank accounts to Russians has also pleaded guilty. The single largest criminal case involves 13 Russians and three Russian companies, accused of conspiring together to fund a hidden but effective social media campaign to exploit American divisions on race and other hot-button topics as well as favor Trump over Clinton.

What have we learned so far?

The criminal cases so far have not resolved the core question of Trump-Russia collusion, but they have revealed a determination by Russians to put Trump in office, and they've exposed the sometimes-shadowy foreign entanglements maintained by Trump aides before, during and after the campaign. They've also underscored how Trump associates were aware of Russian outreach efforts during the campaign and how at least one believed Russia to be in possession of compromising information on Clinton. An indictment against Manafort and Gates, for instance, accused them of working as foreign agents for Ukrainian interests and funneling millions of dollars from the work into offshore accounts used to fund lavish lifestyles. Charging documents in Papadopoulos's case make clear that during the presidential campaign he was told by a professor who claimed powerful connections to the Kremlin that Russia had dirt on Clinton in the form of thousands of emails. And Flynn's guilty plea revealed how, contrary to public assertions from the White House, the incoming national security adviser did indeed discuss sanctions with the then-Russian ambassador, Sergey Kislyak, during the transition period.

What's yet to come?

The biggest unresolved question is whether Trump will sit for an interview with Mueller — and what will happen if he does not. Trump at times has expressed a desire to be questioned by the special counsel team, though his mood toward the investigators soured considerably following an April 9 raid targeting his personal lawyer, Michael Cohen — an offshoot investigation in New York City that poses its own legal peril for the president. Mueller has dozens of questions he'd like to put before the president, largely focused on obstruction but also on the underpinning issue of possible campaign collusion. The legal team initially said it hoped to make a decision on an interview by May 17, the one-year mark, but lawyer Rudy Giuliani told The Associated Press last week that they likely won't decide until after the planned summit with North Korean leader Kim Jong Un next month. If Trump does say yes, look for his legal team to try to aggressively narrow the scope of questioning and limit the time of any sit-down. If he says no, Mueller may take the extraordinary step of seeking a grand jury subpoena to force his testimony, though such a move could prolong the investigation by many months and may end with a fight before the U.S. Supreme Court.

What happens if Mueller finds wrongdoing by the president?

The end game is unclear. Mueller will be expected to report his findings to Deputy Attorney General Rod Rosenstein, the Justice Department official who appointed him. A report of his conclusions may then be passed onto Congress, but it's not clear when or how much of Mueller's work will be revealed to the public. Depending on the outcome, it is possible Congress could use the findings to start impeachment proceedings — especially if Democrats take control in the midterm elections. Justice Department legal opinions from 1973 and 2000 have suggested that a sitting president is immune from indictment and that criminal charges would undermine the ability of the commander in chief to do the job. Mueller would presumably be bound by that conclusion.

What has Mueller said about all of this?