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– The law enforcement investigation into possible coordination between Russia and the Trump campaign has identified a current White House official as a significant person of interest, showing that the probe is reaching into the highest levels of government, according to people familiar with the matter.

The senior White House adviser under scrutiny by investigators is someone close to the president, according to these people, who would not further identify the official.

The revelation comes as the investigation appears to be entering a more overtly active phase, with investigators shifting from work that has remained largely hidden from the public to conducting interviews and using a grand jury to issue subpoenas. The intensity of the probe is expected to accelerate in the coming weeks, the people said.

The sources emphasized that investigators remain keenly interested in people who previously wielded influence in the Trump campaign and administration but are no longer part of it, including former National Security Adviser Michael Flynn and former campaign chairman Paul Manafort.

Flynn resigned in February after disclosures that he had lied to administration officials about his contacts with Russian Ambassador Sergey Kislyak. Current administration officials who have acknowledged contacts with Russian officials include President Donald Trump's son-in-law, Jared Kushner, as well as Attorney General Jeff Sessions and Secretary of State Rex Tillerson.

People familiar with the investigation said the intensifying effort does not mean that criminal charges are near, or that any such charges will result. Earlier this week, Deputy Attorney General Rod Rosenstein appointed former FBI Director Robert Mueller to serve as special counsel and lead the investigation into Russian ­meddling.

It is unclear exactly how Mueller's leadership will affect the direction of the probe, and he is already bringing in new people to work on the team. Those familiar with the case said its significance had increased before Mueller's appointment.

Although the case began quietly last July as an effort to determine whether any Trump associates coordinated with Russian operatives to meddle in the campaign, the investigative work now being done by the FBI also includes determining whether any financial crimes were committed by people close to the president. The people familiar with the matter said the probe has sharpened into something more fraught for the White House, the FBI and the ­Justice Department — particularly because of the public steps investigators know they now need to take, the people said.

When subpoenas are issued or interviews are requested, it is possible the people being asked to talk or provide documents will reveal publicly what they were asked about.

A small group of lawmakers known as the Gang of Eight was notified of the change in the investigation's tempo and focus at a classified briefing Wednesday, the people familiar with the matter said. Then-FBI Director James Comey publicly confirmed the existence of the investigation in March.

Justice Department spokeswoman Sarah Isgur Flores would not "confirm or deny" any of this report. An FBI spokesman declined to comment.

White House spokesman Sean Spicer said, "As the president has stated before, a thorough investigation will confirm that there was no collusion between the campaign and any foreign entity."

While there has been a loud public debate in recent days over the question of whether the president might have attempted to obstruct justice in his private dealings with Comey, whom Trump fired last week, people familiar with the matter said investigators are more focused on Russian influence operations and possible financial crimes.

The FBI's investigation seeks to determine whether and to what extent Trump associates were in contact with Kremlin operatives, what business dealings they might have had in Russia, and whether they in any way facilitated the hacking and publishing of e-mails from the Democratic National Committee and Hillary Clinton's campaign chairman, John Podesta, during the presidential campaign. Several congressional committees also are investigating, though their probes could not produce criminal charges.

A grand jury in Alexandria, Va., recently issued a subpoena for records related to Flynn's business, the Flynn Intel Group, which was paid more than $500,000 by a company owned by a Turkish American businessman close to top Turkish officials, according to people familiar with the matter.

The Flynn Intel Group was paid for research on Fethullah Gulen, a cleric who Turkey's current president believes was responsible for a coup attempt last summer. Flynn retroactively registered with the Justice Department in March as a paid foreign agent for ­Turkish interests.

Separately from Mueller's probe, Flynn is being investigated by the Pentagon's top watchdog for his foreign payments. Flynn also received $45,000 to appear in 2015 with Russian President Vladimir Putin at a dinner for RT, a Kremlin-controlled media organization.

Flynn discussed U.S. sanctions against Russia with Russia's ambassador to the U.S. the month before Trump took office, and he withheld that fact from the vice president. That prompted then-acting Attorney General Sally Yates to warn the White House's top lawyer that Flynn might be susceptible to blackmail.