US President Donald Trump said that he was being investigated for his decision last month to fire the head of the FBI, media reports said.Trump on Friday said the person conducting that probe was the individual who had told him to dismiss James Comey, who had been at the head of the bureau from 2013 until he was fired early last month."I am being investigated for firing the FBI Director by the man who told me to fire the FBI Director! Witch Hunt," Trump said in a Twitter message.He did not mention any names but his message comes a day after he took to Twitter to slam a media report indicating that special counsel Robert Mueller was investigating Trump himself for obstruction of justice related to the Russia probe.Mueller, a former FBI director who was named special counsel for the Russia investigation after Comey was fired, is investigating whether the Trump campaign colluded with Kremlin officials in a bid to tilt the 2016 election in his favour.It is not clear, however, if Trump was referring in Friday's tweet to Mueller or Deputy Attorney General Rod Rosenstein, who wrote a letter to Attorney General Jeff Sessions recommending that the president fire Comey.Comey, whom Trump fired in early May, said in Senate testimony earlier this month that the President spoke to him privately on February 14 and asked him to drop the bureau's investigation into former National Security Adviser Michael Flynn's alleged ties to Russia.Trump denies having told Comey to back off the probe into Flynn, who had resigned on February 13 after it emerged that he had misled Vice President Mike Pence about the content of telephone conversations with the Russian ambassador to the US.