Friday's tweets are the latest in a week of angry social media responses by the President over a report by The Washington Post that Mr Mueller was looking into whether Mr Trump obstructed justice.
It wasn't immediately clear to whom Trump was referring, though the man who wrote a memo recommending Comey's firing, Deputy Attorney General Rod Rosenstein, was also the official who approved the appointment of a special counsel to look into Russian election meddling.
Rosenstein played a role in that decision, his memo critical of Comey, was initially used by the White House to justify the termination.
Mr Rosenstein reportedly told Associate Attorney General Rachel Brand that she would have authority over the probe if he were to remove himself from the investigation.
The Democratic National Committee called on Rosenstein to recuse himself from the Russian Federation matter, but it said authority over the investigation should be given to Mueller and not another Trump appointee.
Rosenstein took over the investigation after Attorney General Jeff Sessions recused himself because he was a Trump campaign surrogate.More news: 26 injured as plane hits turbulence
"Oh, I was gonna fire regardless of recommendation", President Donald Trump said.
The White House confirmed that Mueller was among the candidates Trump interviewed to replace Comey.
Trump's friend Newt Gingrich - who is out with a new book titled "Understanding Trump" - is among those raising questions about the "anti-Trump" focus of Mueller's investigation.
Trump's tweet could force Rosenstein's recusal in the Mueller investigation, according to a Bloomberg View article by Harvard law professor Noah Feldman. The source said investigators have sought information from Richard Ledgett, who recently retired as deputy NSA director and, the source said, had written a memo documenting an attempt from Trump urging Rogers to tamp down on the FBI's Russian Federation investigation.
The department would not comment on the record on whether Trump, who has repeatedly complained about leaks on the case, requested the statement.More news: Champions Trophy : India bowl against Pakistan in final
During an appearance on NBC "Meet the Press", Sekulow said plainly that Trump was not under investigation, directly contradicting a tweet from the president last week.
Lieu isn't the only House Democrat warning Trump of consequences for firing Mueller and Rosenstein. Rep.
The special counsel is also investigating the finances and businesses of Trump's senior advisers and his son-in-law Jared Kushner, The Washington Post reported.More news: Castile protesters target city where officer worked
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