In the effort led by President Trump to create a misleading impression of widespread voter fraud, administration and campaign officials have seized on nine mail-in military ballots in a Pennsylvania county that Mr. Trump won by 20 points in 2016.
Federal officials have disclosed that they are investigating whether local elections officials improperly discarded the ballots, at least seven of which were cast for Mr. Trump, they said. A Justice Department official said on Friday that Attorney General William P. Barr briefed Mr. Trump this week on the case.
The disclosure of the investigation’s existence was highly unusual and came as Mr. Trump has ramped up his false assertions that widespread mail-in voting is rife with fraud. It prompted elections and legal experts to express fears that political appointees were using the levers of law enforcement to undermine voters’ confidence in the results of the election.
“There is a battle here about the narrative in fraud and voting, and it looks like there’s a continued effort to gather as much evidence as possible to give them any little scraps for that narrative,” said Samuel W. Buell, a criminal law professor at Duke University School of Law.
Regardless of Mr. Barr’s intentions in briefing Mr. Trump, Mr. Buell said, the attorney general and the president discussing an ongoing criminal investigation created a perception that they might be acting improperly, especially because the situation involved such a small number of ballots in a state where six million people are expected to vote.
The issue in Pennsylvania emerged on Thursday and centered on mail-in ballots cast in Luzerne County, in the eastern part of the state.
According to a timeline released on Friday by county officials, a contractor who started working for the county this month to process ballots had discarded several cast by members of the military into the office’s trash. It was unclear why.
Working with the F.B.I., local officials sifted through three days’ worth of trash this month in search of the ballots, uncovering the nine, according to the Justice Department.
The county, which is primarily controlled by Republicans, said on Friday that it had been unaware of whom the ballots were cast for until the Justice Department released the information.
Some of the ballots were said to be missing envelopes, and it was not clear whether they could be counted. The Pennsylvania Supreme Court recently sided with the Trump campaign in a lawsuit seeking to reject all ballots that arrived without the requisite secrecy envelope, which are known as naked ballots.
The president and his allies rolled out the information about the ballots throughout the day on Thursday, in a highly unusual public relations campaign.
It began when Mr. Trump gave an interview to Brian Kilmeade of Fox News radio in the morning, complaining about mail-in voting and sharing details that appeared to match the inquiry in Pennsylvania.
“These ballots are a horror show,” the president said. “They found six ballots in an office yesterday, in a garbage can. They were Trump ballots, eight ballots in an office yesterday, in a certain state. And they had Trump written on it, and they were thrown in a garbage can. This is what is going to happen, this is what is going to happen, and we are investigating that. It’s a terrible thing that is going on with these ballots — who is sending them?”
A little more than an hour later, the F.B.I. director, Christopher A. Wray, undercut the president’s narrative during congressional testimony. Mr. Wray told lawmakers that the bureau had not uncovered a “coordinated national voter fraud effort.”
Early that afternoon, the White House again emphasized the missing ballots. At a news briefing, Kayleigh McEnany, the press secretary, confirmed that discarded Trump ballots had been found in Pennsylvania.
“I believe you should be getting more information on that shortly,” she said.
Not long after, the United States attorney for central Pennsylvania, David J. Freed, released a detailed statement about the ballots and said that the F.B.I. was investigating. He later issued a second version revising details about how many ballots were cast for the president.
Then the Trump campaign pointed to the statement as evidence of mail-in voter fraud.
“BREAKING: FBI finds military mail-in ballots discarded in Pennsylvania. 100% of them were cast for President Trump. Democrats are trying to steal the election,” Matt Wolking, a campaign spokesman, tweeted.
The White House quickly echoed the campaign’s messaging.
“This is concerning to say the least,” Judd Deere, a White House spokesman, tweeted. “President @realDonaldTrump is the only one fighting for a free and fair election.”
Mr. Trump again discussed the ballots with reporters on the White House lawn on Thursday as he left for a campaign rally in North Carolina.
“We want to make sure the election is honest, and I’m not sure that it can be,” he said. “I don’t know that it can be with this whole situation — unsolicited ballots. They’re unsolicited, millions being sent to everybody. And we’ll see.”
On Friday morning, Mark Meadows, the White House chief of staff, was asked in an interview on CBS about the discrepancy between the president’s language and Mr. Wray’s testimony.
“With all due respect to Director Wray, he has a hard time finding emails in his own F.B.I., let alone figuring out whether there’s any kind of voter fraud,” Mr. Meadows said in an apparent reference to the agency’s handling of the Russia investigation.
Legal experts said the attorney general briefing the president about such a small number of ballots was akin to telling him about the most mundane law enforcement developments across the country.
“This would be like the president keeping track of how many passengers were stopped at LAX coming in from outside the country who had contraband in their luggage,” Mr. Buell said, referring to Los Angeles International Airport.
“Normally, the White House would be briefed on a criminal D.O.J. investigation only if it was of some major importance to national security,” he added. “This isn’t just the kind of thing that usually bubbles its way up to the A.G. or the president in normal circumstance.”
Katie Benner contributed reporting.