As special prosecutor Robert Mueller racks up one indictment after another in his probe of Russian influence in the 2016 presidential election, his investigators are increasingly focusing on a seemingly insignificant fact concerning the president: When exactly did private citizen Donald Trump decide he should run for president?

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FOX Business has learned that Mueller’s team has been focusing on this detail in interviews with people who have been called to provide testimony in the probe into Russian meddling in the 2016 election and whether Trump aides or associates violated federal laws by colluding in that effort to gain an edge over Hillary Clinton.

People with direct knowledge of these interviews said that Mueller’s investigators have made finding the details of the timing of Trump’s decision a key question for people in his orbit who have been asked to speak with the special prosecutor’s office.

Why exactly the special prosecutor’s office cares so much about the timing of Trump’s decision to run is unclear, but people with knowledge of the investigation said such information may be used to show that the future president entered the race at about the same time Russian agents began to expand efforts to influence the election in his favor, first over his defeated GOP challengers in the primaries and then when he pulled off an upset victory over Clinton in November 2016.

Trump has said the Russian meddling as outlined in Mueller’s recent indictments of 13 Russian nationals was not connected to his campaign because the charges show that the efforts began in 2014, “long before” he decided to run. Trump officially announced he was running for president in June 2015 at a lavish ceremony at Trump Tower in New York City.

The indictment charged these people with violating American laws by engaging in a “conspiracy to defraud the United States,” largely by pushing a pro-Trump message beginning in 2014, using phony social media accounts and other measures.

Witnesses who have appeared before Mueller’s investigators have contradicted Trump’s timeline. They say Trump had flirted with running for president as far back as 2011 and began to consider running seriously after GOP nominee Mitt Romney lost to President Obama in 2012, people with knowledge of the matter said.

These people have told Mueller’s investigators that by 2014, Trump was committed to running for the 2016 presidential election.

Trump’s attorney Ty Cobb didn’t return emails and calls for comment. A spokesman for Mueller declined to comment.

Nailing down the exact date that Trump began to run for the 2016 election, of course, is just one facet of Mueller’s wide-ranging probe into Russian interference in the election and whether people associated with the Trump campaign colluded in that effort.

Trump has steadfastly denied any collusion despite the wide-ranging probe, which has ensnarled numerous past and present Trump associates. As FOX Business was first to report, Mueller’s team interviewed Sam Nunberg, a former Trump campaign and one of the president’s earliest political advisers, last week.

Nunberg began working for Trump in 2011 as an aide to another Trump political adviser, Roger Stone. An attorney for Nunberg didn’t respond to a telephone call for comment; Stone declined to comment.

During these interviews, Mueller’s team appears interested in establishing a timeline that starts with Trump’s various attempts to do business in Russia and stretches through Trump’s decision to run for president. The team is looking at any contacts his campaign or people in his orbit have had with Russian government officials or associates.

Mueller’s agents, for example, have asked various witnesses about the president’s past business dealings in Russia, including his since-scrapped plans to build a Trump-branded hotel in Moscow, according to people with direct knowledge of the matter.

Mueller’s investigators have also asked about Trump and his organization’s ties with various Russian businessman, including those with purported links to Russian organized crime. They have asked whether Russian businessmen could be seen inside Trump’s executives office in New York City’s Trump Tower.

Investigators have also asked whether people around the president had ties to WikiLeaks, the anti-secrecy group that hacked into Democratic National Committee computers and released damaging emails about Clinton during the 2016 election, these people added.

Democrats including Clinton have accused WikiLeaks and its founder, Julian Assange, of working with the Russian operatives to help Trump win the election. Assange has denied the accusation.

Mueller was appointed special prosecutor by Deputy Attorney General Rob Rosenstein in May of last year after Trump fired the original investigator of the alleged campaign abuse: FBI Director James Comey.

In addition to interviewing people in the Trump orbit, Mueller has indicted a number of Trump associates and reached guilty pleas with others, such as Michael Flynn, the former national security adviser, and former campaign aides George Papadopoulos and Rick Gates.

But none of those people pleaded guilty to collusion—a point the president has made time and time and again in publicly attacking the probe. For example, Mueller’s indictment of the 13 Russian nationals made clear no member of the Trump campaign was directly involved in their efforts. Flynn and Papadopoulos pleaded guilty to lying to the FBI about contacts in Russia, not collusion, while Gates tendered a guilty plea over charges of money laundering.

The same goes for the recent indictment of Paul Manafort on charges of money laundering when he was doing business in Ukraine with pro-Russian associates.

Manafort has vowed to fight the indictment, calling the allegations an “untrue piled up charges.”

More from FOX Business Mueller investigators to question former Trump aide Sam Nunberg in Russian probe

People close to Trump have called the Mueller probe and its various facets a fishing expedition to find some shred of proof of wrong doing that simply isn’t there. “He’s trying to get something because he has nothing,” said one Trump legal representative who asked not to be quoted by name.

But some legal observers speculate that Mueller is methodically building a case that will show the myriad of ties between the Trump campaign and his business interests with Russian businessmen and associates of President Vladimir Putin and that the president’s firing of Comey was intended to obstruct the FBI’s efforts to cover those ties.

As the investigation swirls, the president has grown increasingly agitated, lashing out at Democrats as they push to widen congressional inquiries into Russian election meddling, and even his own Justice Department for allowing Mueller’s appointment.

As FOX Business previously reported, Trump was told by his legal team late last year that Mueller’s probe would be wrapped up by now, only to discover that the special prosecutor’s witness list and indictment count continues to grow.

Earlier Tuesday, Trump tweeted out that the investigation is a “Witch Hunt!” after news broke that White House Communication Director Hope Hicks will be called before a congressional committee investigating the matter.