Prosecutors consider Robert Mueller's investigation is going deeper as White House communications director Hope Hicks, a key witness to Trump's controversies, is about to be interviewed.

Hicks is known to be a reliable witness, especially in dealing with the allegations of Russia interfering with Hillary Clinton's campaign during the 2016 presidential election.

Former Clinton Administration lawyer, Adam Goldberg, told Politico:

Any time you can get someone who is the right-hand person or who’s been around the primary target of an investigation, under oath, answering detailed questions, [that] means you’ve progressed very far along in the investigation.

Hicks's association with the president is crucial for Mueller's team because of her insight into Trump's habits and being present for his various reactions and responses about colluding with Russia.

According to former White House prosecutors, the investigation is close to the end of the probe when a senior-level aide like Hicks is next for interviewing.

Politico said that people close to Hicks, who first worked for Ivanka Trump in 2012, was preparing months in advance to appear before Mueller's team, but an official date is yet to be determined.

One friend told Politico that Hicks is expected to be cooperative.

I think she’s smart enough and sensible enough that she knows she’d be doing no one any favors by lying — the best thing she can do for everyone is, to tell the truth.

On Wednesday, White House attorney Ty Cobb said that all current aides will have been interviewed by the special counsel "shortly after Thanksgiving."

Nothing about the White House’s commitment to fully cooperate with the special counsel, including doing necessary interviews, has changed. We continue to be in a full cooperation posture.

It's worth noting that Hicks worked under former Donald Trump campaign advisor Paul Manafort, who was recently indicted for charges including money laundering over $75 million in off-shore accounts. In April 2016, Manafort allegedly advised Hicks to dismiss questions regarding his dealings with a Russian aluminum magnate, Oleg Deripaska, an ally of Vladimir Putin.

She was also present at the Washington Post editorial board meeting when Donald Trump announced the foreign policy advisory team that included George Papadopoulos, who just admitted to lying to the FBI about dealings with Russian officials.

Another crucial moment Hicks presided over includes a discussion that Trump had about a letter he wrote with White House senior policy adviser and top campaign official, Stephen Miller, to justify firing former FBI director James Comey.

Twitter is embracing itself for the impending interview.

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H/T - politico, newsweek, twitter