WASHINGTON — President Trump escalated his criticism of the F.B.I. on Friday over its investigation of possible links between Russia and his campaign, adding a new round of his own complaints to a growing conservative effort to discredit the inquiry.

“It’s a shame what’s happened with the F.B.I.,” the president told reporters on the South Lawn of the White House before departing for an event at the F.B.I. Academy in Quantico, Va. “It’s a very sad thing to watch.”

Without citing specifics, Mr. Trump described an extraordinary “level of anger” at the F.B.I. over the investigation. He labeled “disgraceful” recently released text messages between one of the agents on the investigation and a lawyer for the bureau who were critical of him. The special counsel, Robert S. Mueller III, removed the agent from the investigation when he learned of the texts.

Mr. Trump’s latest attacks on the F.B.I. kept alive a rare public feud between a president and the nation’s premier law enforcement agency, friction that could be seen as Mr. Trump undermining faith in the integrity of an inquiry he has long derided as a “witch hunt.” They also came amid a campaign by congressional Republicans, conservative media and the president’s own lawyers to paint the inquiry as a partisan effort to weaken the president.