President Trump fumed to aides and longtime friends last week after learning his Twitter feed had become a point of interest in special counsel Robert Mueller’s obstruction of justice probe, according to sources close to the White House, some of whom said it was the angriest Trump had been since FBI agents raided the office of his personal attorney earlier this spring.

The revelation that Mueller and his team were looking through critical statements Trump had made on the social media platform about his own attorney general and former FBI Director James Comey came as a surprise to West Wing officials. They sometimes worry about the controversial nature of the president’s tweets, but they say they never saw them as potential evidence for a criminal investigation.