Special counsel Robert Mueller is homing in on President Donald Trump’s favorite mode of communication to determine whether, coupled with the President’s private pressuring of Justice Department officials about the Russia probe, he obstructed justice, The New York Times reported Thursday.

According to three people briefed on the matter who spoke to the Times, Mueller is looking at Trump’s tweets and disparaging public comments about Attorney General Jeff Sessions and former FBI Director James Comey. Mueller hopes to determine whether the combination of Trump’s firing of Comey and his private and public pressuring of his own DOJ to squash the Russia probe could amount to the President obstructing justice.

Trump’s lawyers say that the actions Mueller is investigating on the obstruction front were well within Trump’s executive authority. Rudy Giuliani even told the Times that it’s not possible to obstruct justice in “public.”

“If you’re going to obstruct justice, you do it quietly and secretly, not in public,” he said.

According to the Times, Trump’s lawyers are privately anxious that Mueller will use the tweets and private conversations to build a case that shows a far-reaching pattern of attempts to interfere in the investigation.