President Trump speaks with reporters before boarding Marine One en route to the G20 Summit from the South Lawn of the White House on Thursday. (Jabin Botsford/The Washington Post)

President Trump took to Twitter for a fourth straight morning on Thursday to attack special counsel Robert S. Mueller III’s investigation into Russian election interference, questioning whether it will “just go on forever” and calling it a waste of money.

“Did you ever see an investigation more in search of a crime?” Trump wrote in a tweet. He also asserted that the probe “has proven only one thing-there was NO collusion with Russia.”

Mueller’s 18-month-old investigation has led to charges against 32 people, including 26 Russians. Four aides to Trump have pleaded guilty to various charges.

The president’s tweets came just a few hours before Michael Cohen, a former personal attorney to Trump, pleaded guilty in New York to lying to Congress about a Moscow real estate project that Trump and his company pursued during the months he was running for president.

As he left the White House on Thursday, Trump told reporters that Cohen was “weak” and said he is cooperating with Mueller to get a reduced sentence on unrelated charges.

“He’s got himself a big prison sentence, and he’s tried to get a much lesser prison sentence by making up a story,” Trump said.

Many of Trump’s attacks on Mueller Thursday were familiar. The intensity has picked up considerably in recent days in the wake of Trump’s submission of written answers to Mueller and several related developments in the news.

Mueller is continuing to investigate whether Trump’s campaign coordinated with Russia in the 2016 U.S. presidential campaign and whether Trump has obstructed the probe.

In Thursday’s tweets, Trump complained again that Mueller is not investigating his 2016 Democratic opponent, Hillary Clinton, and other political adversaries. Without evidence, he accused them of committing “atrocious, and perhaps subversive, crimes.”

In an interview with the New York Post on Wednesday, Trump said that a presidential pardon for Paul Manafort, his former campaign chairman, was “not off the table.”

“It was never discussed, but I wouldn’t take it off the table,” Trump told the publication. “Why would I take it off the table?”

Manafort pleaded guilty in September, on the eve of jury selection for his trial in Washington, to two charges: conspiring to defraud the United States and conspiring to obstruct justice. He admitted to years of financial crimes related to his undisclosed lobbying work for a pro-Russian political party and politician in Ukraine before joining Trump’s campaign.

Prosecutors working with Mueller this week accused Manafort of breaching a plea agreement by repeatedly lying to them in their investigation into Russian interference. In a court filing, Manafort denied doing so intentionally.

The New York Times reported this week that despite agreeing to cooperate with Mueller, Manafort’s lawyer continued to brief Trump’s legal team.

“It’s very sad what’s happened to Paul,” Trump told reporters as he left the White House. “I’ve never seen anybody treated so badly.”