The investigation into Russian involvement in the 2016 presidential election continues – with Special Counsel Robert Mueller at its helm.

Mueller, 73, took over the federal government’s probe into alleged collusion between President Trump’s campaign and Russian officials in May 2017. Already, his investigation has led to charges for four U.S. citizens and indictments against more than a dozen Russian nationals.

Trump has in the past expressed willingness to testify under oath as part of Mueller’s investigations and has repeatedly denied any “collusion” with Russians.

Why is Mueller overseeing the Russia investigation?

The Department of Justice announced the appointment of Mueller to oversee the federal investigation into Russia’s alleged meddling in the 2016 election in May 2017.

The appointment came after a growing cry – mostly from Democrats – for someone outside the Justice Department to handle the probe. Attorney General Jeff Sessions had already recused himself from the investigation. 

Mueller led the FBI through the Sept. 11, 2001, terrorist attacks and served under presidential administrations of both parties.

For the inquiry into the 2016 election, Mueller has the authority to prosecute any crimes uncovered during this investigation, and he was given wide authority to investigate whether Trump or his associates colluded with the Kremlin to win the White House.

Has anyone been charged?

In his leading role, Mueller took over an ongoing investigation into Paul Manafort’s financial dealings in Ukraine.

Manafort, Trump’s former campaign chairman, and Richard Gates were indicted on Oct. 27 on multiple counts, including: conspiracy against the U.S., conspiracy to launder money, false statements and failure to file reports of foreign bank and financial accounts. Manafort and Gates pleaded not guilty to the initial charges.

Nearly four months later, on Feb. 22, the pair were hit with additional tax evasion and bank fraud charges. The additional charges involve much of the same conduct Manafort and Gates were initially charged with, but they increase the amount of money Manafort is accused of laundering through offshore accounts to $30 million.

Michael Flynn, the administration’s short-lived national security adviser, was charged in December with lying to the FBI about certain conversations he had with a Russian ambassador. He pleaded guilty. 

Additionally, George Papadopoulos pleaded guilty in 2017 to one count of making false statements to investigating FBI agents, according to court documents. Papadopoulos was a foreign policy adviser for Trump’s campaign.

Alex van der Zwaan, an attorney who allegedly lied to investigators in the Russia inquiry, was charged in federal court in February and pleaded guilty a few days later. According to charging documents, van der Zwaan was employed by a law firm hired by the Ukraine Ministry of Justice in 2012. He admitted to lying to investigators about his interactions with Gates. 

Three Russian entities and 13 Russian nationals were indicted by a federal grand jury on Feb. 16 for allegedly interfering in the election. Mueller’s case alleges those involved had a sophisticated plot to wage “information warfare” on the U.S.

However, the Justice Department did not say the actions had an impact on the outcome of the election. Deputy Attorney Gen. Rod Rosenstein said, “There is no allegation in this indictment that any American was a knowing participant in this illegal activity.”

What’s this we keep hearing about a controversy with Mueller’s staff?

The Trump administration has sharply criticized Mueller’s investigation, as several of his attorneys on staff donated to Democratic campaigns, including to Trump’s 2016 rival, Hillary Clinton.

Additionally, two FBI officials – Peter Strzok and Lisa Page – are under fire for the anti-Trump text messages they exchanged during the election. Strzok was part of Mueller’s team but was removed after the text messages were revealed.

What has Trump said about Mueller’s investigation?

Trump has oftentimes dismissed the allegations that he colluded with Russia during the election. He said he is “looking forward” to eventually  being questioned under oath by Mueller.

“I just hope the final determination is a truly honest one…”

– President Trump

He’s said the allegations are a “fake story that is demeaning to all of us and most of all demeaning to our country and demeaning to our Constitution.”

“I just hope the final determination is a truly honest one, which is what the millions of people who gave us our big win in November deserve and what all Americans who want a better future want and deserve,” Trump said at a rally in West Virginia last year.

The president also warned Mueller to stay within certain boundaries as he investigates.

Trump and Mueller have sent messages “back and forth,” according to Trump’s outside counsel. A spokesman for Mueller told Fox News that the messages have been “very professional.”

Fox News’ Madeline Farber and The Associated Press contributed to this report.