Federal prosecutors on Thursday added major accusations to an indictment charging former President Donald J. Trump with mishandling classified documents after he left office, presenting evidence that he told the property manager of Mar-a-Lago, his private club and residence in Florida, that he wanted security camera footage there to be deleted.

The new accusations were revealed in a superseding indictment that named the property manager, Carlos De Oliveira, as a new defendant in the case. He is scheduled to be arraigned in Miami on Monday.

The original indictment filed last month in the Southern District of Florida accused Mr. Trump of violating the Espionage Act by illegally holding on to 31 classified documents containing national defense information after he left office. It also charged Mr. Trump and Walt Nauta, one of his personal aides, with a conspiracy to obstruct the government’s repeated attempts to reclaim the classified material.

The revised indictment added three serious charges against Mr. Trump: attempting to “alter, destroy, mutilate, or conceal evidence”; inducing someone else to do so; and a new count under the Espionage Act related to a classified national security document that he showed to visitors at his golf club in Bedminster, N.J.

The updated indictment was released on the same day that Mr. Trump’s lawyers met in Washington with prosecutors in the office of the special counsel, Jack Smith, to discuss a so-called target letter that Mr. Trump received this month suggesting that he might soon face an indictment in a case related to his efforts to overturn the results of the 2020 election. It served as a powerful reminder that the documents investigation is ongoing, and could continue to yield additional evidence, new counts and even new defendants.

Prosecutors under Mr. Smith had been investigating Mr. De Oliveira for months, concerned, among other things, by his communications with an information technology expert at Mar-a-Lago, Yuscil Taveras, who oversaw the surveillance camera footage at the property.

That footage was central to Mr. Smith’s investigation into whether Mr. Nauta, at Mr. Trump’s request, had moved boxes in and out of a storage room at Mar-a-Lago to avoid complying with a federal subpoena for all classified documents in the former president’s possession. Many of those movements were caught on the surveillance camera footage.

The revised indictment said that in late June of last year, shortly after the government demanded the surveillance footage as part of its inquiry, Mr. Trump called Mr. De Oliveira and they spoke for 24 minutes.

Two days later, the indictment said, Mr. Nauta and Mr. De Oliveira “went to the security guard booth where surveillance video is displayed on monitors, walked with a flashlight through the tunnel where the storage room was located, and observed and pointed out surveillance cameras.”

A few days after that, Mr. De Oliveira went to see Mr. Taveras, who is identified in the indictment as Trump Employee 4, and took him to a small room known as an “audio closet.” There, the indictment said, the two men had a conversation that was meant to “remain between the two of them.”

It was then that Mr. De Oliveira told Mr. Taveras that “‘the boss’ wanted the server deleted,” the indictment said, referring to the computer server holding the security footage.

Mr. Taveras objected and said he did not know how to delete the server and did not think he had the right to do so, the indictment said. At that point, the indictment said, Mr. De Oliveira insisted again that “the boss” wanted the server deleted, asking, “What are we going to do?”

Two months later, after the F.B.I. descended on Mar-a-Lago with a search warrant and hauled away about 100 classified documents, people in Mr. Trump’s orbit appeared to be concerned about Mr. De Oliveira’s loyalties.

“Someone just wants to make sure Carlos is good,” the indictment quoted Mr. Nauta as saying to another Trump employee.

In response, the indictment said, that employee told Mr. Nauta that Mr. De Oliveira was “loyal” and “would not do anything to affect his relationship with Mr. Trump.” After the conversation, Mr. Trump — who during his 2016 presidential campaign often assailed his opponent, Hillary Clinton, for deleting material from her email server — called Mr. De Oliveira and said that he would get him a lawyer.

The revised indictment also charges Mr. De Oliveira with lying to federal investigators. It recounts an exchange in which he repeatedly denied seeing or knowing anything about boxes of documents at Mar-a-Lago, even though, the indictment said, he had personally observed and helped move them when they arrived.

Mr. De Oliveira’s lawyer, John Irving, declined to comment.

A statement attributed only to the Trump campaign called the new accusations a “desperate and flailing attempt” by the Justice Department to undercut Mr. Trump, the current front-runner for the Republican nomination to take on President Biden next year.

Mr. Trump and Mr. Nauta have both pleaded not guilty to the charges in the original indictment. Their case has been scheduled to go to trial in May.

The new charges lay out in detail efforts by Mr. Nauta to speak with Mr. De Oliveira about the security camera footage and to determine how long the footage was stored after the government sought to obtain it under a subpoena.

The indictment contains an additional charge related to a classified document — a battle plan related to attacking Iran — that Mr. Trump showed, during a meeting at his Bedminster golf club, to two people helping his former White House chief of staff Mark Meadows write a book.

The updated indictment provides specific dates during which Mr. Trump was in possession of the document — from Jan. 20, 2021, the day he left office, through Jan. 17, 2022, the date Mr. Trump turned over 15 boxes of presidential material to the National Archives. The specificity of the dates indicates that prosecutors have the document in question and the indictment describes it as a “presentation concerning military activity in a foreign country,” adding it was marked top secret.

The meeting at which Mr. Trump showed off the document was captured in an audio recording and Mr. Trump can be heard rustling paper and describing the document as “secret” and “sensitive.”

Still, he has tried to suggest that he never had a document in his hand and was simply blustering.

“There was no document,” Mr. Trump claimed to the Fox News host Bret Baier in a recent interview. “That was a massive amount of papers and everything else talking about Iran and other things. And it may have been held up or may not, but that was not a document. I didn’t have a document per se. There was nothing to declassify.”

The original indictment filed by Mr. Smith and his team in June came about two months after local prosecutors in New York filed more than 30 felony charges against Mr. Trump in a case connected to a hush money payment made to a porn star in advance of the 2016 election.

Mr. Trump remains under investigation by Mr. Smith’s office over his wide-ranging efforts to retain power after his election loss in 2020, and how those efforts led to the Jan. 6, 2021, assault on the Capitol by a pro-Trump mob. He is also being scrutinized for possible election interference by the district attorney’s office in Fulton County, Ga.

Chris Cameron and Charlie Savage contributed reporting.

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