After months of pulling his punches with Robert Mueller, President Donald Trump launched an extraordinary offensive against the special counsel on Friday that shows no signs of slowing down. “The Mueller probe should never have been started in that there was no collusion and there was no crime,” Trump tweeted Saturday, one in a series of increasingly frantic messages in which he called the Russia investigation a “witch hunt”; accused the F.B.I., Justice Department, and State Department of “leaking, lying, and corruption”; claimed that former F.B.I. director James Comey had perjured himself; and alleged that the Mueller investigation is fatally compromised. “Why does the Mueller team have 13 hardened Democrats, some big Crooked Hillary supporters, and Zero Republicans?” Trump wrote on Sunday. “Does anyone think this is fair? And yet, there is NO COLLUSION!” On Monday morning, the president tweeted simply: “A total WITCH HUNT with massive conflicts of interest!”

The inciting incident appeared to have been the firing of Deputy F.B.I. Director Andrew McCabe, who was dispatched by Attorney General Jeff Sessions on Friday, hours before he was set to retire with his full pension. Trump was jubilant, celebrating the political assassination as “a great day for the hard working men and women of the F.B.I.” and “a great day for democracy.” The president barely disguised the fact that McCabe’s ouster—ostensibly the result of an internal investigation that found the deputy director had been less than truthful under questioning about a leak to the press—was related to his work on the Russia probe. “Sanctimonious James Comey was his boss and made McCabe look like a choirboy. He knew all about the lies and corruption going on at the highest levels of the F.B.I.!” Trump tweeted. His personal attorney, John Dowd, followed up by calling on Deputy Attorney General Rod Rosenstein, who is overseeing Mueller’s investigation, to “bring an end to alleged Russia collusion investigation manufactured by McCabe’s boss James Comey based upon a fraudulent and corrupt dossier.”

Behind the scenes, however, it seems that Trump’s rage had a more ominous trigger. The president was already on edge earlier last week, when it was reported that Mueller had subpoenaed the Trump Organization, the latest in a flurry of moves that suggest the special counsel is digging into the president’s personal finances and those of his family—territory that Trump has suggested would constitute a “violation” of Mueller’s mandate. But the more immediate cause for the weekend outburst appears to have been a letter from Mueller’s team, reported Saturday by The New York Times, that included a list of questions related to the special counsel’s efforts to interview with Trump. Sources told the Times that the questions are meant to serve as a starting point for a sit-down examination under oath. The same day, Axios reported that Mueller had interviewed McCabe about Comey’s firing, and revealed that McCabe, like his former boss, had also written contemporaneous memos documenting his interactions with the president.

Trump, emboldened by Dowd’s statements in the press, unleashed a torrent of abuse at McCabe, arguing that the entire Russia investigation was “based on fraudulent activities and a Fake Dossier paid for by Crooked Hillary and the DNC”; claiming that McCabe “never took notes when he was with me” (“can we call them Fake Memos?”); and suggesting that McCabe’s wife’s Democratic political connections had influenced the probe. “How many hundreds of thousands of dollars was given to wife’s campaign by Crooked H friend, Terry M, who was also under investigation? How many lies? How many leaks? Comey knew it all, and much more!” (Michael Bromwich, McCabe’s lawyer, shot back that Trump’s tweets “confirm that he has corrupted the entire process that led to Mr. McCabe’s termination and has rendered it illegitimate.”)