The special counsel, Robert S. Mueller III, is scrutinizing tweets and negative statements from the president about Attorney General Jeff Sessions and the former F.B.I. director James B. Comey, according to three people briefed on the matter.

Several of the remarks came as Mr. Trump was also privately pressuring the men — both key witnesses in the inquiry — about the investigation, and Mr. Mueller is examining whether the actions add up to attempts to obstruct the investigation by both intimidating witnesses and pressuring senior law enforcement officials to tamp down the inquiry.

Mr. Mueller wants to question the president about the tweets.