U.S. President Donald Trump continued to fume over the Russia investigation on Sunday, May 10, more than a year after special counsel Robert Mueller filed his report without recommending charges against the president but only three days after the justice department said it would drop its case against Michael Flynn, Trump’s first national security adviser, the Guardian reports.

“The biggest political crime in American history, by far!” the president wrote in a tweet accompanying a conservative talk show host’s claim that former U.S. President Barack Obama “used his last weeks in office to target incoming officials and sabotage the new administration”.

The tweet echoed previous messages retweeted by Trump, which earned rebukes for relaying conspiracy theories. On Sunday afternoon the president continued to send out a stream of tweets of memes and rightwing talking heads claiming an anti-Trump conspiracy. One tweet by Trump simply read: “OBAMAGATE!”

Trump fired Flynn, a retired general, in early 2017, for lying to Vice-President Mike Pence about conversations with the Russian ambassador regarding sanctions levied by the Obama administration in retaliation for interference in the 2016 election.

The US intelligence community has long held that such efforts were meant to tip the election towards Trump and away from Hillary Clinton, the Democratic nominee.

Flynn pleaded guilty to lying to the FBI – which Trump has acknowledged – and co-operated with Mueller, who was appointed to take over the investigation of Russian interference after Trump fired FBI director James Comey.

Mueller did not establish a criminal conspiracy but did lay out extensive links between Trump and Moscow and instances of possible obstruction of justice by the president.

Flynn sought to change his plea while awaiting sentencing and the president championed his case, floating a possible pardon. On Thursday, in an act that stunned the US media, attorney general William Barr said the justice department would drop the case entirely.