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The Senate testimony of ex-FBI boss James Comey dominated the headlines last week, but the latest announcements from Robert Mueller's special counsel investigation could be a more ominous indication of trouble on the horizon for the Trump administration.

Mr Mueller, who was tasked by Deputy Attorney General Rod Rosenstein with overseeing the Justice Department's inquiry into possible ties between the Trump presidential campaign and Russian operatives, is staffing up his office - and bringing in some prosecutorial heavy-hitters.

The hires could be an indication of the direction of the probe and the seriousness with which Mr Mueller is taking the enterprise.

Consider Michael Dreeben. On Friday afternoon the National Law Journal reported that the deputy solicitor general, a criminal-law expert who has argued more than 100 cases before the US Supreme Court, would be joining the special counsel team on a part-time basis.

"Mueller's selection of Dreeben suggests that the special counsel is looking very carefully into whether criminal laws were broken by the Trump campaign and the president's associates," writes Cristian Farias of the Huffington Post.

Lawfare blog's Paul Rosenzweig called news of the hiring "the worst thing that happened to Donald Trump this week".

"He is quite possibly the best criminal appellate lawyer in America," he continued.

Then there's Andrew Weissmann, head of the fraud section of the Justice Department's Criminal Division, who joined the team in late May. He is perhaps best known for his investigation into Volkswagen scheme to bypass emissions requirements on some of its autos. From 2002 to 2005 Weissmann oversaw the Enron energy company inquiry that ended in the prosecution and imprisonment of its top executive, Kenneth Lay.

If Mr Mueller's investigation touches on the president's business interests, including allegations of Russian financial entanglements, Weissmann would be the kind of experienced legal hand best suited for the job.

Those with a sense of history might have noted that Mr Mueller also tapped James Quarles, a private lawyer who was part of the Watergate Special Prosecution Force in the 1970s and is well aware of the challenges of investigations that tread close to the presidency.

Image copyright Getty Images Image caption Newt Gingrich has apparently changed his mind about Robert Mueller's investigation

Jeannie Rhee, a former deputy attorney general, and Aaron Zebley, Mr Mueller's chief of staff when he was FBI director, help round out the special counsel team that has taken up residence in a Justice Department office building in downtown Washington.

The moves haven't gone unnoticed by Mr Trump's supporters. On Monday morning former Speaker of the House Newt Gingrich, who has fashioned himself as an expert on all things Trump, tweeted a shot across the independent counsel's bow.

"Republicans are delusional if they think the special counsel is going to be fair," he wrote. "Look who he is hiring."

He then suggested readers check out the Federal Election Committee records of some of Mr Mueller's staff, which reveals four of them - Dreeben, Rhee, Weissmann and Quarles - had given money to past Democratic presidential nominees.

The counter to that argument is that Mr Mueller himself was appointed to Justice Department posts, including FBI director, by Republican presidents.

Media playback is unsupported on your device Media caption Trump's love-hate relationship with Comey over a tumultuous year

Mr Gingrich's criticism could be a harbinger of a drive by Trump surrogates to undermine Mr Mueller's authority and, perhaps, lay the groundwork to question his ultimate prosecutorial decisions or seek his removal. It marks a rather stark change of views for the former House speaker, who just a month ago tweeted that Mr Mueller was a "superb choice" as special counsel.

One of Mr Trump's personal lawyers, Jay Sekulow, demurred when asked on Sunday if the president could fire Mr Mueller.

"The president is going to seek the advice of his counsel and inside the government as well as outside," he said. "And I'm not going to speculate on what he will or will not do."

And the speculation built further on Monday evening, when a close friend of the president, Christopher Ruddy, told PBS Newshour he was "considering perhaps" firing Mr Mueller.

Donald Trump has called the ongoing investigation a "witch hunt", a "hoax" and an excuse by Democrats for why they lost the presidential race. Mr Mueller's moves, however, indicate he does not share Mr Trump's view. He's assembling a team built for the long haul, with the talent and experience to take cases to trial and, if necessary, send people to prison.

So far, Mr Trump has only directed his criticism at former FBI Director James Comey. It may only be a matter of time before some of that attention is directed at Mr Mueller, however.