The Attention Was All on Mar-a-Lago. Some of the Action Was at Bedminster.

For all the attention focused during the investigation into former President Donald J. Trump’s handling of classified documents on Mar-a-Lago, his private club and residence in Florida, another of Mr. Trump’s properties has played a crucial, if quieter, role in the case: his 520-acre golf club in Bedminster, N.J.

Mar-a-Lago grabbed headlines last August after federal agents descended on the compound and hauled away a trove of more than 100 classified documents, and the pictures of boxes of presidential records piled there — including in a bathroom — helped explain why prosecutors chose to indict him this month.

But Bedminster, where Mr. Trump spends his summers, has turned out also to have been a focus of investigators, a flashpoint in the conflict between prosecutors and Mr. Trump’s lawyers, and the scene of a central episode in Mr. Trump’s indictment: a meeting in which he was recorded showing off what he described as a “highly confidential” plan to attack Iran.

That audio recording, which was published on Monday by The New York Times, was the latest piece of evidence placing Bedminster on an almost equal footing with Mar-a-Lago as a key location in the case being pursued against Mr. Trump by the special counsel Jack Smith. Previously unreported details of the investigation show that prosecutors working for Mr. Smith have subpoenaed surveillance footage from Bedminster, much like they did from Mar-a-Lago, and fought a pitched battle with Mr. Trump’s lawyers late last year over how best to search the New Jersey property.

At one point in the early fall of last year, investigators went so far as to discuss executing a search warrant at Bedminster, according to two people briefed on the matter. Investigators were concerned that more documents were stashed at the club and the only way to account for them was to search the property. But one of the people said the Justice Department lacked probable cause to obtain a warrant from a judge.

The discussions about the warrant took place around the time that Jay Bratt, the top counterintelligence official at the Justice Department, told Mr. Trump’s legal team that prosecutors believed Mr. Trump still had more classified materials in his possession.

Mr. Trump acquired Bedminster in 2002 and uses it as a seasonal escape from both New York and southern Florida. The property’s role as a summer getaway made a cameo appearance in the indictment filed by Mr. Smith: Prosecutors said that Mr. Trump’s co-defendant in the case, Walt Nauta, loaded boxes from Mar-a-Lago onto a plane “that flew Trump and his family north for the summer” on the same day that Mr. Bratt showed up in Florida to collect all of the classified documents remaining there.

As for the recording of Mr. Trump, it was made at Bedminster in July 2021 during a meeting attended by two of his aides — identified by people with knowledge of the matter as Margo Martin and Liz Harrington, who sat in on some of Mr. Trump’s book interviews that summer — as well as by a publisher and writer working on a memoir for Mark Meadows, Mr. Trump’s final White House chief of staff.

On the recording, Mr. Trump can be heard rustling through papers and describing for his guests a “secret” plan regarding Iran that he said had been drawn up by Gen. Mark A. Milley, the chairman of the Joint Chiefs of Staff, and the Defense Department. Mr. Trump was describing the document in an effort to rebut an account that General Milley feared having to keep him from manufacturing a crisis with Iran in the period after Mr. Trump lost his re-election bid in late 2020.

“This totally wins my case, you know,” Mr. Trump says, adding that the papers he was apparently displaying were “highly confidential” and “secret.”

One of the women heard speaking on the recording was Ms. Harrington, three people with knowledge of the matter said. Ms. Harrington, one of Mr. Trump’s most aggressive defenders on Twitter, did not respond to questions about whether she is one of the voices talking on the recording as Mr. Trump appears to show a piece of paper.

Ms. Harrington; Ms. Martin, who worked for Mr. Trump in the White House; and the other participants in the meeting could be important witnesses if Mr. Trump’s case goes to trial, since they can provide firsthand descriptions of what he was showing as he discussed the Iran plan. A lawyer for Ms. Martin declined to comment.

People close to Mr. Trump have suggested that the recording does not specify whether Mr. Trump actually showed a confidential document to anyone — and he told Bret Baier of Fox News last week that there was “no document.” Yet the indictment states plainly in its first few pages that he did display a document, an assertion that appears to be backed up by his own words as captured by the recording.

On Tuesday, in an interview with Fox News, Mr. Trump did not repeat the claim that there was no document, but maintained that he had done nothing wrong.

“I said it very clearly — I had a whole desk full of lots of papers, mostly newspaper articles, copies of magazines, copies of different plans, copies of stories, having to do with many, many subjects, and what was said was absolutely fine,” Mr. Trump said, when asked how he squares the recording with what he told Mr. Baier. “I don’t do things wrong. I do things right. I’m a legitimate person.”

Steven Cheung, a spokesman for Mr. Trump, said the full context of the recording showed the former president did “nothing wrong at all.”

Shortly after the meeting at Bedminster, people in Mr. Trump’s orbit were aware something unusual had happened, according to a person with knowledge of the events.

The indictment describes Mr. Trump meeting with the people working on the book and members of his staff, “none of whom possessed a security clearance.” It also says that Mr. Trump “showed and described a ‘plan of attack.’”

Mr. Meadows’s book contains a reference to a document that he claimed Mr. Trump said General Milley had typed himself.

“The president recalls a four-page report typed up by Mark Milley himself,” Mr. Meadows’s book said. “It contained the general’s own plan to attack Iran, deploying massive numbers of troops, something he urged President Trump to do more than once during his presidency. President Trump denied those requests every time.”

People close to General Milley have denied that he urged attacking Iran.

Mr. Smith’s indictment suggests that prosecutors have obtained a large amount of surveillance camera footage from Mar-a-Lago, some of it showing Mr. Nauta, Mr. Trump’s co-defendant and personal aide, moving boxes in and out of a storage room in the basement of the compound.

The movement of those boxes — undertaken at Mr. Trump’s request, prosecutors say — lies at the heart of a conspiracy charge accusing Mr. Trump and Mr. Nauta of obstructing the government’s efforts to reclaim all of the classified materials that Mr. Trump took with him from the White House.

But prosecutors also issued at least one subpoena for surveillance camera footage from Bedminster as well, according to two people familiar with the matter. The subpoena for that footage came some time after the government’s request for the Mar-a-Lago footage, the people said, though it remains unclear what the footage shows or precisely why prosecutors wanted to obtain it.

One thing, however, is for certain: Even after the F.B.I. searched Mar-a-Lago, Mr. Bratt and his team remained concerned that Mr. Trump was still holding on to classified documents in violation of a subpoena for them that the government had issued three months earlier. So prosecutors contacted Mr. Trump’s representatives in September — one month after the Mar-a-Lago search — to give the former president yet another chance to return any relevant material, according to sealed court papers described to The Times.

At first, Mr. Trump’s lawyers refused to conduct further searches of his properties or provide a sworn statement certifying that everything had been turned over, the court papers say, according to a person briefed on their contents. In making their refusal, the lawyers questioned the scope and validity of the initial subpoena, and argued that Mr. Trump’s presidential office might incriminate itself if more classified documents were discovered and returned.

The government responded by filing a motion to compel compliance with the original subpoena, and a hearing was scheduled for late October in front of Judge Beryl A. Howell, who was then the chief judge in Federal District Court in Washington.

One of Mr. Trump’s former lawyers, Timothy Parlatore, has since suggested that Boris Epshteyn, another lawyer close to Mr. Trump, “attempted to interfere” with searches commissioned by the Trump legal team around that time. Mr. Parlatore made the remarks about Mr. Epshteyn in a CNN interview last month in which he cited his differences with Mr. Epshteyn as a key reason that he had resigned from representing Mr. Trump. (He later said Mr. Epshteyn did not commit any “wrongdoing” and called it a disagreement.)

Minutes before the hearing in front of Judge Howell, Mr. Parlatore alerted her and the government that a team of professionals with military training had searched Bedminster for classified materials just days earlier. They were supervised by another one of Mr. Trump’s lawyers at the time, James Trusty.

But the Justice Department, according to court papers described by people familiar with them, was not impressed. Prosecutors complained that the search had been limited to certain areas of Bedminster and was not accompanied by a sworn statement detailing which parts of the club had been examined.

In the end, Judge Howell decided in favor of the government, ordering Mr. Trump’s lawyers to provide a sworn statement about which parts of Bedminster had been searched. She also told the lawyers to make a “custodian of records” for Mr. Trump’s presidential office available to testify about the search in front of a grand jury.

Ben Protess, William K. Rashbaum and Adam Goldman contributed reporting.