"Current and former leaders in the intelligence community have repeatedly said there's been no effort to impede the investigation in any way", Ronna Romney McDaniel, chairwoman of the Republican National Committee, said in a statement.
Mueller brought in James Quarles and Jeannie Rhee from his previous firm WilmerHale, and Andrew Weissmann, the leader of the Enron fraud investigation.
Since his appointment on May 17, Mueller, a former Federal Bureau of Investigation director, has recruited a high-powered team of lawyers, investigators and prosecutors with deep expertise in areas ranging from financial fraud to constitutional law.
Mr Mueller is hiring for the team which is probing a range of issues related to interference in last year's USA election.
Kushner and other Trump business associates are being looked at in the investigation, according to a new report.
In the report by the Washington Post, the investigation is being expanded to consider whether President Trump obstructed justice by asking former Federal Bureau of Investigation director James Comey to end the probe into former #national security Adviser, Michael Flynn, over his ties with Russian Federation, and the firing of the former FBI boss. He is reportedly interviewing three top intelligence officials - Director of National Intelligence Dan Coats, National Security Agency Director Mike Rogers and retired deputy NSA director Richard Ledgett - as part of the probe as early as this week.
Coats and Rogers told a Senate panel a week ago they did not feel pressured by Trump to intervene in the case, but declined to say what Trump asked them to do. They have no idea what evidence the FBI has, and until the investigation is completed and charges are either filed or not filed, they won't - can't - have any idea.
US officials familiar with the investigation told The Washington Post about its financial focus Thursday. "Nice", the president tweeted just before 4 a.m.
He decried a "witch hunt" that he said is being "led by some very bad and conflicted people!"
The interest in Kushner's activities by the special counsel's office is no more extensive than the FBI's interest in his multiple roles on both the Trump campaign and transition team, which CNN reported last month.
"The FBI leak of information regarding the President is outrageous, inexcusable and illegal", Mark Corallo, the spokesman for Trump's legal team, said in an email. He was one of the architects of Trump's recent foreign trip, which especially in its visits to Saudi Arabia and Israel went smoothly and mostly gaffe-free.
Despite suggestions by a Trump friend in a television interview that the president was weighing dismissing Mueller as special counsel, the White House declared late Tuesday that there was no such plan.
"I think he's weighing that option", he said of Trump.
The idea that this is a witch hunt, as Trump childishly tweeted this morning, is getting more and more laughable as each passing day brings about another questionable tie or secret dealing with Russian Federation taken by this president or those in his orbit.