President Trump said Friday his campaign was “conclusively” spied on, calling it “a really bad situation” and that it amounts to “treason.”

“My Campaign for President was conclusively spied on. Nothing like this has ever happened in American Politics. A really bad situation. TREASON means long jail sentences, and this was TREASON,” the president tweeted.

The Federal Bureau of Investigation sent an investigator posing as a campaign staffer into Mr. Trump’s campaign due to intelligence that Russians were attempting to influence the 2016 election, The New York Times reported earlier this month.

Attorney General William P. Barr said during a Senate Appropriations Committee hearing that he thought “spying did occur,” telling the Senate, “The question is whether it was adequately predicated.”

The attorney general later clarified when asked about the basis for his suspicions.

“OK. I am concerned about it, and I was asked about whether there was any basis for it. I believe there is a basis for my concern, but I’m not going to discuss the basis,” Mr. Barr said.