"I am being investigated for firing the FBI Director by the man who told me to fire the FBI Director! Witch Hunt"

US President Donald Trump tweeted from the White House shortly before leaving for Miami.

As Bob Mueller, the special investigator now heading the FBI's probe into allegations of Russian election meddling, filled out his legal team and prepared to begin interviews, Trump erupted furiously for a second consecutive day.

Trump's digital volley appeared to be trained at Deputy Attorney General Rod Rosenstein, who recommended the firing last month of James Comey -- an almost unprecedented move that could yet imperil Trump's presidency.

A source on Trump's legal team claimed the president was referring to a Washington Post story "from the other night" rather than confirming he is being investigated.

The Washington Post was one of several media that revealed this week that Trump himself is being investigated on allegations of obstructing justice.