However, a number of investigators opted to purchase extra protection as the probe moves further into examining President Donald Trump's finances and those of his associates, a source familiar with the investigation told CNN.
On Thursday, after reports of the grand jury surfaced, the White House said in a statement provided by spokeswoman Sarah Sanders that Comey "said three times the President is not under investigation and we have no reason to believe that has changed".
But the grand jury news - first reported by The Wall Street Journal - suggests Mueller may be expanding his investigation into following the money toward possible Trump campaign and business transactions with Russians. He has also suggested that he might attempt to fire Mueller, a former FBI director whose investigation falls under the purview of the Department of Justice. Prosecutors do not seek the assistance of the grand jury's subpoena power, and do not contemplate presenting evidence to a grand jury, unless they see a realistic possibility of filing criminal charges.
According to Deputy Attorney General Rod Rosenstein's letter appointing Mueller, he is authorized to investigate any links or coordination between the Trump campaign and the Russian government and "any matters that arose or may arise directly from the investigation".
The news of a grand jury comes up as the US President prepares to take a 17-day-vacation to New Jersey later this week.
Tributes pour in for late actress Jeanne Moreau
Said the actress herself, in an interview with Film Comment magazine, "I was at that age where one lives very egocentrically". Her most famous role came in Jules And Jim , in which her iconic laugh and modern style became immortalised in film posters.
Donald Trump Jr and the president's son-in-law and White House adviser, Jared Kushner, could also be called to testify over their controversial June 2016 meeting with a Russian lawyer.
CBS News has learned that special counsel Robert Mueller is using a federal grand jury - in part to issue subpoenas to compel documents and information as part of his Russian Federation investigation, reports CBS News' Andres Triay and Pat Milton. He argued that "what the prosecutors should be looking at are Hillary Clinton's 33,000 deleted emails". The White House has every interest in bringing this to a prompt and fair conclusion.
"Trump is probably excited because he thinks a grand jury is just a fancier jury", Meyers joked, before launching into his impersonation of the president.
NBC's Seth Meyers began: "Donald Trump's presidency was already in a tailspin before today's bombshell news that the special counsel has impaneled a grand jury in the Russian Federation investigation". But it is a laughable distraction that will dissuade neither Mueller's team, nor the American people. Reacting to some reports that Trump may fire Mueller, two bipartisan groups of senators introduced bills Thursday to make it harder for the President to do so. How will Trump react to the grand jury? "I just hope the final determination is a truly honest one, which is what the millions of people who gave us our big win in November deserve and what all Americans who want a better future want and deserve", he said. As witnesses begin to appear and the media coverage intensifies, he will undoubtedly lash out more harshly, the way he did last night in West Virginia.