Even as former President Donald J. Trump invoked the Fifth Amendment in a New York inquiry into his business practices, he and his allies began an effort to discredit another investigation, suggesting without evidence that federal agents may have planted incriminating materials before searching his residence in Florida on Monday.

In a post on Wednesday morning on his social media app, Truth Social, Mr. Trump said that the agents who showed up at Mar-a-Lago would not let anyone, including his lawyers, observe their work, which was authorized by a federal magistrate judge.

Mr. Trump — who has not released the search warrant provided by the agents to his lawyers or the manifest his team was given of the materials the agents gathered and removed — baselessly suggested that the F.B.I. might have acted inappropriately.

“Everyone was asked to leave the premises, they wanted to be alone,” he wrote, “without any witnesses to see what they were doing, taking or, hopefully not, ‘planting.’”

Mr. Trump’s effort to sow doubts about the F.B.I.’s search before the results are known was in keeping with his history of trying to head off news by seeking to turn the tables on his accusers. The former president’s first line of attack against adversaries is often to mount a campaign in advance to discredit them and their work.

Around the same time that his message appeared on social media, The New York Post published an article about the F.B.I. search, noting in the third paragraph that “a source close to the former president” had expressed concern that federal agents or prosecutors could have “planted stuff” in the 128-room building.

The unfounded allegations seem to have first emerged on Tuesday morning when Christina Bobb, a lawyer for Mr. Trump who was present during the search, was asked during an interview on the right-wing TV station Real America’s Voice about whether it was possible that federal agents could have planted evidence.

“There is no security that something wasn’t planted,” Ms. Bobb said before quickly admitting that she did not know if anything improper had occurred.