He appeared to be referring to Rod Rosenstein, deputy attorney general who is now leading the probe.
While Mr Rosenstein appointed Mr Mueller as a special counsel to oversee the investigation, it is possible that he could be called as a witness as part of the investigation. "Sad!" Mr. Trump said on Twitter.
Mr Putin echoed Mr Trump's criticism of Mr Comey, saying it was "very strange" a former Federal Bureau of Investigation chief leaked details of his conversations with the USA president to the media through a friend.
But it was released shortly after The Washington Post reported - citing unnamed U.S. officials familiar with the matter - that the business dealings of President Donald Trump's son-in-law, Jared Kushner, are being investigated by Justice Department special counsel Robert Mueller, who has been investigating Russian meddling in the 2016 election and the Trump administration's potential ties to Russia.
ABC News reported that Mr Rosenstein has told colleagues privately that he may have to step aside from the investigation. Earlier this week, news reports indicated that Trump was thinking about firing Mueller but was dissuaded by staff.
"I'm going to acknowledge we had a meeting with the special counsel Mueller, but I'm not going to get into the contents", Mr Mark Warner, the ranking Democrat on the Senate Intelligence Committee, told reporters later. Rosenstein told the Senate on May 17 that he knew Comey would be fired before he wrote the memo.
The New York Times also reported that Mueller was seeking interviews with the three officials.
Trump took to his Twitter account Thursday morning to denounce the report.
The tweets immediately prompted concerns among lawmakers that Trump might try to can both Mueller and Rosenstein.
"I faced great pressure because of Russian Federation".