We break down Special Counsel Robert Mueller’s investigation into alleged Russian meddling in the 2016 elections.

A federal jury in the US has charged 13 Russian nationals and three Russian companies with conspiracy to defraud the US, as part of an investigation into alleged interference in the US 2016 presidential elections.

The charges stem from Special Counsel Robert Mueller’s probe into alleged Russian meddling.

The indictment comes just days after President Donald Trump’s former chief strategist Steve Bannon was questioned by Mueller’s team.

Last month, Jeff Sessions, the US attorney general, was also questioned by Mueller for hours.

In December, Michael Flynn, the former US national security adviser, became the fourth known person connected to the Trump campaign to be charged in connection with the investigation on possible Russian meddling.

The probe began in May to investigate any links or coordination between the Russian government and individuals associated with the Trump campaign.

Al Jazeera answers some of the key questions surrounding the probe and those involved.

What is the Robert Mueller Russia-Trump investigation?

Robert Mueller, a lawyer and the head of the FBI from 2001 to 2003, was appointed as special counsel to the US Justice Department to investigate possible Russian meddling in the 2016 US presidential electionon May 17. {articleGUID}

Mueller was directed to investigate “any links and/or coordination between the Russian government and individuals associated with the campaign of President Donald Trump”, as well as “any matters that arose or may arise directly from the investigation”, according to Assistant Attorney General Rod Rosenstein’s letter announcing the appointment.

A “special counsel” is a lawyer appointed from outside the government to investigate possible wrongdoings of senior officials in the US. Otherwise, lawyers within the government could be tasked with investigating their superiors, including the sitting a president or attorney general, leading to a conflict of interest.

Mueller and his team have been working since May to see if there are links between Russian authorities and the Trump campaign.

These connections were suspected after numerous revelations regarding Russia’s role in the hacking of the Democratic National Convention’s email database, undisclosed meetings between Trump’s son-in-law and senior adviser Jared Kushner and Michael Flynn’s undisclosed work for foreign governments.

So far, four people connected to the Trump campaign have been charged in the probe.

The first charges were announced in October. Paul Manafort, Trump’s former campaign manager, and his business associate Rick Gates, were charged on 12 counts, which included conspiracy against the US, money laundering and other financial charges. It was also revealed in October that George Papadopoulos, an adviser during the campaign, pleaded guilty to “making false statements to the FBI”.

In December, former National Security Advisor Michael Flynn pleaded guilty to making false statements to the FBI over his contact with the Russian ambassador before Trump took office. He became the first person who served in the White House to be charged.

In February, 13 Russian nationals and three Russian companies were charged as part of the investigation.

What are the 13 Russians charged with?

Mueller announced the indictment of 13 Russian nationals and three Russian companies in February. They are charged with interfering in the 2016 presidential election.

in the 2016 presidential election. The charges are described as a conspiracy to sway public opinion and promote the campaign of Trump, against Democratic rival Hillary Clinton.

According to court documents, a Russian organisation called the Internet Resarch Agency LLC, “sought, in part, to conduct what it called ‘information warfare against the United States of America’ through fictitious US personas on social media platforms and other Internet-based media”.

The documents add that the Russian organisation wanted to “sow discord in the US political system, including the 2016 US presidential election”.

The Internet Research Agency was allegedly led by Yevgeny Prigozhin, who was also named as a defendant . Prigozhin is a wealthy associate of Russian President Vladimir Putin.

. Prigozhin is a wealthy associate of Russian President Vladimir Putin. “By mid-2016, [the] defendants’ operations included supporting the presidential campaign of then-candidate Donald J Trump (‘Trump Campaign’) and disparaging Hillary Clinton,” the indictment reads.

According to the indictment, those allegedly involved posed as Americans online, posted adverts against Clinton and organised pro-Trump political rallies within the US.

involved posed as Americans online, posted adverts against Clinton and organised pro-Trump political rallies within the US. “The indictment alleges that the Russian conspirators want to promote discord in the United States and undermine public confidence in democracy,” Deputy Attorney General Rod J Rosenstein said in a statement following the announcement of the charges.

of the charges. The interference first began in 2014, according to the indictment, the year before Trump announced his candidacy for the presidency.

“From in or around 2014 to the present, [the] defendants knowingly and intentionally conspired with each other (and with other persons known and unknown to the Grand Jury) to defraud the United States by impairing, obstructing and defeating the lawful functions of government,” the indictment said.

Following the indictment of the Russians, Trump defended his campaign, saying his staff “did nothing wrong”. He reiterated that there was “no collusion”.

Trump also falsely tweeted that he never said Russia didn’t interfere. “I never said Russia did not meddle in the election, I said ‘it may be Russia, or China or another country or group, or it may be a 400-pound genius sitting in bed and playing with his computer’,” Trump said. “The Russian ‘hoax’ was that the Trump campaign colluded with Russia – it never did!” he added. But according to fact-checking website PolitiFact, Trump has said multiple times that the allegations is a “made-up story”.

Russian Foreign Minister Sergey Lavrov dismissed allegations Moscow meddled in the elections. “As long as we don’t see facts, everything is blather,” Lavrov said.

Who is Michael Flynn and what did he plead guilty to?

Michael Flynn is a retired US Army lieutenant general who served in the military for over three decades, leaving the service in 2014.

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Flynn was a highly decorated general who over oversaw top secret operations and helped shaped US strategy against armed groups.

Flynn also served as an adviser for the Trump campaign and later as the national security adviser.

The retired general rose to prominence in the 2016 campaign for being an ardent critic of the handling of counterterrorism operations by the administration of former President Barack Obama.

the administration of former President Barack Obama. After clashing with Obama-era officials over the danger of armed groups during his tenure as the head of the Defense Intelligence Agency – the highest-ranking military intelligence position – in 2014, Flynn was fired and subsequently retired.

Flynn resigned as national security adviser less than a month after Trump took office after reports emerged that said that the Trump administration was warned by the Department of Justice that Flynn’s communication with the Russians could leave the president in a compromised position.

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After leaving the military, Flynn started a consulting firm, Flynn Intel Group Inc, with his son, Michael G Flynn. This is where his known work for foreign governments began.

His firm had been working for a Dutch company with ties to the Turkish government. While working for the Trump campaign, Flynn reportedly attended a meeting in 2016 with Turkish officials where a plan to abduct Fethullah Gulen, a Turkish Islamic leader living in the US who Turkey accuses of plotting the failed coup, was discussed. Flynn’s lawyer said the reports were “false”.

US law requires that officials working in the interest of foreign governments register as foreign agents. Flynn did so in March 2017.

On December 1, Flynn pleaded guilty to making false statements to the FBI in relation to his contact with the Russian ambassador just before Trump took office.

Flynn said he was directed by a senior member of the Trump campaign.

According to the court documents, Flynn:

Falsely told the FBI on or about December 29, 2016, that he did not ask the then-Russian ambassador to “refrain from escalating the situation in response to sanctions that the United States had imposed against Russia that same day”. The documents also say that Flynn “did not recall the Russian ambassador subsequently telling him that Russia had chosen to moderate its response to those sanctions as a result of his request”.

Falsely told the FBI on or about December 22, 2016, that he did not ask the then-Russian ambassador to “delay the vote on or defeat a pending United Nations Security Council Resolution; and that the Russian ambassador subsequently never described to Flynn Russia’s response to his request”.

Who are Paul Manafort and Rick Gates and what have they been charged with?

Paul Manafort was Trump’s campaign chair and Rick Gates was his deputy. Both were charged on October 27 with 12 counts including conspiracy against the US, making false statements, money laundering and undisclosed foreign lobbying.

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Manafort has more than 40 years of experience in politics and lobbying. He, along with Trump ally Roger Stone and Gates, worked at the DC-based consulting firm Black, Manafort, Stone and Kelly.

A lawyer by trade, Manafort joined the Trump campaign in March 2016 and was appointed chairman of the campaign in June 2016.

Manafort was reportedly forced out of the Trump campaign team in August 2016 after coming under fire for allegedly taking millions of dollars of undisclosed payments for lobbying efforts on behalf of the pro-Russia Ukrainian political party, Party of Regions, which was overthrown in the 2014 Ukrainian revolution.

Manafort worked as a consultant for Yanukovych and his party, maintaining an office in Kiev. Ledgers found there showed that Manafort had been paid more than $12m by the Party of Regions.

Gates joined Manafort when he went to work for the Trump campaign. His past involvement with Republican politics included working for Senator John McCain’s 2008 presidential campaign.

The two men turned themselves into the FBI on October 30.

Manafort and Gates have pleaded not guilty to the charges and have been released $10m and $5m bail, respectively.

According to the indictment, Manafort and Gates hid “tens of millions of dollars” from their work in Ukraine from 2006 through at least 2016 by laundering “the money through scores of United States and foreign corporations , partnerships and bank accounts”.

, partnerships and bank accounts”. The charges had been expected after the FBI seized documents from Manafort’s home in August.

Much of the media attention on Manafort focuses on a meeting he had with Trump’s eldest son, Donald Trump Jr, his son-in-law, Jared Kushner, and a Russian government lawyer, who claimed to have incriminating information against Hillary Clinton, Trump’s Democratic challenger in the last year’s election.

In February, Mueller filed new charges against Manafort and Gates. The 32-count indictment includes charges of bank fraud and making false statements on tax returns.

Who is George Papadopoulos?

George Papadopoulos, who worked for the Trump campaign, pleaded guilty to making false statements to the FBI in October.

Papadopoulos was one of Trump’s foreign policy advisers, according to Trump in his interview with the Washington Post’s editorial board.

During the interview, Trump said Papadopoulos was “an energy and oil consultant, an excellent guy”.

The Chicago native of Greek background is 30 years old. He began his political career in December 2015 as a national security and foreign policy adviser in Ben Carson’s 2016 presidential campaign.

After Carson dropped out of the race and endorsed Trump, becoming a close ally, Papadopoulos was hired by the Trump campaign in March 2016.

Papadopoulos offered to arrange various meetings with Russian officials and the Trump campaign during his tenure as an adviser, according to US media.

The court document outlining Papadopoulos’s false statements says he lied about contact with an “overseas professor” with extensive ties to the Russian government who informed the foreign policy adviser that Russians possessed “dirty” information on then-presidential candidate Hillary Clinton.

After his arrest and guilty plea, Trump and his press officer Sarah Huckabee Sanders attempt to downplay Papadopoulos’s role in the campaign. Trump said he was a “young, low-level volunteer named George, who has already proven to be a liar”.

What has Trump said?

Trump had previously attempted to downplay meddling by attacking former campaign officials and Hillary Clinton, sometimes alleging she was connected to Russia through donations to the Clinton Foundation made by Frank Giustra, an investor in Canadian company Uranium One, which also sells uranium to the Russian government. The claim has not been proven.

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Trump directly addressed the issue with reporters in the press cabin on the Air Force One in early November.

“[Putin] said he didn’t meddle … I asked him again. You can only ask so many times,” he said. The comments came after Trump informally spoke with Putin during on the sidelines of the Asia-Pacific Economic Cooperation forum in Danang, Vietnam.

Trump then appeared to somewhat contradict himself when asked whether he believed a CIA assessment that Russia did play a role the US elections. He said: “As to whether I believe it or not, I’m with our agencies, especially as currently constituted.”

A CIA report in January said that Russian President Vladimir Putin “ordered” a campaign to influence the election.

Trump has tweeted and said several times that the Mueller probe is a “political witch-hunt”.

Following the announcement of charges against Manafort and Gates, Trump tweeted t hat “this is years ago, before Paul Manafort was part of the Trump campaign”. However, the charges relate to dealings in 2016 and 2017.

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He added that there “is no COLLUSION!” – a statement he has repeatedly said in reference to the probe into Russia’s alleged meddling.

White House Press Secretary Sarah Sanders told reporters that the charges “has nothing to do with the president, has nothing to do with the president’s campaign or campaign activity”.

After Michael Flynn pleaded guilty to making false statements, Trump’s lawyer Ty Cobb said in a statement that “nothing about the guilty plea or the charge implicates anyone other than Mr Flynn”.

Cobb added: “The conclusion of this phase of the Special Counsel’s work demonstrates again that the Special Counsel is moving with all deliberate speed and clears way for a prompt and reasonable conclusion.”

The next day, Trump said there was “absolutely no collusion” between his presidential campaign and Russia.

“What has been shown is no collusion, no collusion, there has been absolutely no collusion,” Trump told reporters.

He later tweeted that that Flynn’s actions during the transition “were lawful” and Flynn was fired “because he lied to the vice president and to the FBI”.

In December, Trump said he was not planning on firing Mueller as special counsel. Speculation had been mounting for months that Trump may sack Mueller. When asked by reporters about the anticipated step, Trump said: “No, I’m not”. He went on to say that there was “no collusion whatsoever” between the Trump campaign team and Russian officials.

After the indictment of the 13 Russians and three Russian companies were announced, Trump went on a tweet firestorm, defending his campaign and reiterating that there has been “no collusion”.

He also tweeted: “If it was the GOAL of Russia to create discord, disruption and chaos within the US then, with all the committee hearings, investigations and party hatred, they have succeeded beyond their wildest dreams. They are laughing their a**es off in Moscow. Get smart America!”

Following Trump’s tweets in February, Democrats, as well as some within Trump’s own party, criticised the president and said he “must come around” that the Russians did interfere. Many also said the Obama administration should have done more to prevent the interference .

What has Russia said?