Deputy Attorney General Rod Rosenstein appointed Mueller as special counsel to lead the Russian Federation probe shortly after Comey was sacked in May.
Though Trump was told by Comey on multiple occasions that he was not under investigation by the FBI, the president reportedly became the focus of Mueller's investigation after firing Comey.
The five anonymous sources cited by the Post said three intelligence officials have already agreed to interviews with Mueller's team: Director of National Intelligence Daniel Coats, NSA chief Mike Rogers, and Rogers' recently departed deputy, Richard Legett. The outlet also said the probe now also includes a look at any financial crimes by Trump associates.
Accusations of obstruction arose last month when the US President fired FBI Director James Comey.
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Other factors such as obesity and consumption of high quantities of salt may also play a role in increasing risk of early death . The National Potato Council however adds that potatoes as such are not the dietary culprits regardless of how they are consumed.
But the tweet appeared to verify a report from the Washington Post on Wednesday that the federal criminal probe into the Trump campaign's possible Russian Federation collusion has expanded into investigating Trump for possible obstruction of justice.
Mr Comey, who had been leading one of several Russian Federation inquiries, testified to Congress last week that Mr Trump had pressured him to drop the inquiry into former National Security Adviser Michael Flynn.
Trump turned 71 years old on Wednesday.
Trump's Twitter response followed days of intensifying criticism of Mueller from some conservatives, who have charged the former Federal Bureau of Investigation director and his team with political bias and have claimed his relationship to Comey and an earlier meeting with Trump amount to conflicts of interest. Nevertheless, the Post story is a blockbuster, if only because it should silence the Trump camp's long-held insistence that the president has never been personally under investigation for wrongdoing.
In written testimony to the committee, Mr Comey said that one of the reasons he did not assert this publicly, as requested by the president, was that in the future he might have been faced with the possibility of correcting that assertion.