While President Trump made headlines this week for his provocative rhetoric on North Korea and the Obama administration's nuclear deal with Iran, several major developments related to the ongoing probe of possible collusion between his associates and Russia flew quietly under the radar.

Each new disclosure about the direction and breadth of special counsel Robert Mueller's investigation lent credence to what many legal experts have been saying since the former FBI director began hiring lawyers with expertise in corruption, foreign bribery, and white collar crime: This is serious, and some in Trump's orbit should be worried.

"Combined with a flurry of stories about subpoenas, grand-jury appearances and other activity, it's reasonable to expect that Mueller is moving forward on a number of different fronts and is getting close to entering a litigation phase," Brookings Institution fellows Susan Hennessey and Benjamin Wittes wrote earlier this week.

Here are seven key signs that indicate Mueller and his high-powered team are digging deep and may be nearing the "litigation phase," as Hennessy and Wittes put it.

1. Indictment warning

During the FBI's raid of an apartment belonging to Paul Manafort earlier this summer, Mueller explicitly told the former Trump campaign chairman he "planned to indict him," the New York Times reported Monday. According to the report, Manafort and several FBI agents picked the lock of Manafort's home in the predawn hours of July 26 and left with "binders stuffed with documents" and photographs of "expensive suits in his closet."

The report shed light on the aggressive tactics Mueller has employed as he reportedly seeks to "flip" Manafort against several other persons of interest in the special counsel's investigation, including the president.

2. Record requests

It was widely reported this week that Mueller has begun requesting exhaustive records from White House aides who were clued in to the decision-making process that led to former FBI director James Comey's firing in May, and aware of Trump's response when he learned that ex-national security adviser Mike Flynn was under federal investigation for his lobbying for foreign governments.

Beyond Comey and Flynn, the Washington Post claimed Mueller demanded that any correspondence related to the following individuals or incidents be turned over to his team: Donald Trump Jr.'s meeting with a Russian attorney last June; Trump's Oval Office meeting with Russian Foreign Minister Sergei Lavrov; Manafort; the FBI's interview of Flynn shortly after the inauguration; and a statement issued by former White House press secretary Sean Spicer the night Comey was fired.

3. Spicer's notes and texts

Spicer made news Thursday for being snappish with Axios co-founder Mike Allen during an exchange about the notes he kept during his time in Trump's circle, which could become a valuable resource for Mueller.

"From a legal standpoint I want to be clear: Do not email or text me again. Should you do again I will report to the appropriate authorities," Spicer said in an email, later adding that he would "contact the appropriate legal authorities to address [Allen's] harassment" if he continued to receive requests for comment.

A source close to Spicer said the exchange, which the former press secretary later apologized for, showed how current and former White House officials are trying to be "careful … and want to avoid attracting attention to themselves" as long as the Russia investigation is ongoing.

Spicer is one of several former and current Trump aides whom Mueller has expressed an interest in interviewing, ABC News reported earlier this month. The others include: former chief of staff Reince Priebus, White House communications director Hope Hicks, White House counsel Don McGahn, senior associate counsel James Burnham, and White House spokesman Josh Raffel.

4. Surveillance of Manafort

A bombshell report by CNN late Monday night alleged that U.S. government officials wiretapped former Trump campaign chairman Paul Manafort, who remains at the center of Mueller's investigation, before the November election and during the subsequent transition phase. The surveillance was conducted after federal investigators obtained a so-called FISA warrant, which almost always requires the demonstration of probable cause.

Sources told CNN that "some of the intelligence collected includes communications that sparked concerns among investigators that Manafort had encouraged the Russians to help with the campaign."

5. Grand jury testimony

Mueller is beginning to bring in lobbyists and public relations consultants who have worked with Manafort to testify before a federal grand jury in Washington. One such individual – Manafort spokesman Jason Maloni – did so last week at a courthouse in downtown D.C.

Maloni, who began working for Manafort after the 2016 election, testified for more than two-and-a-half hours, telling reporters on his way in that "hell yeah" he was ready to appear before the grand jury. It was not immediately clear what investigators sought from Maloni's testimony, though the longtime public relations executive may have been aware of potential errors on Manafort's foreign-agent filing amendments, which he retroactively disclosed in June.

Mueller also issued a grand jury subpoena in August to Melissa Laurenza, a lawyer and former National Republican Senatorial Committee staffer who represented Manafort until recently.

6. Facebook ads

The social media giant turned over detailed records to congressional committees and Mueller's team this week regarding ads purchased by a Russian company during the 2016 election. Facebook CEO Mark Zuckerberg confirmed earlier this month that the company bought approximately $3,000 ads between June 2015 and May 2017, totalling $100,000.

There is a good chance Mueller will probe whether the advertisements "showed any of the kind of sophisticated targeting that might indicate that Americans had provided assistance," the Atlantic's David Graham wrote earlier this week.

7. James Quarles

Former assistant special prosecutor for the Watergate investigation James Quarles is the latest to emerge as a central figure assisting with Mueller's probe. The Daily Beast reported Tuesday that Quarles was added to the team as its "point person" for interactions with the White House, noting that he has been constantly in touch with Trump's legal aides to check in on document requests and confirm the status of upcoming interviews.

Quarles left his position as a partner at the global law firm WilmerHale to join Mueller's team in June.