Special counsel Jack Smith's team has questioned several witnesses about a closet and a so-called "hidden room" inside former President Donald Trump's residence at Mar-a-Lago that the FBI didn't check while searching the estate in August 2022, sources familiar with the matter told ABC News.
As described to ABC News, the line of questioning in several interviews ahead of Trump's indictment last year on classified document charges suggests that -- long after the FBI seized dozens of boxes and more than 100 documents marked classified from Trump's Mar-a-Lago estate -- Smith's team was trying to determine if there might still be more classified documents there.
MORE: Attorney warned Trump 'it's going to be a crime' if he didn't comply with subpoena for classified docs: SourcesAccording to sources, some investigators involved in the case came to later believe that the closet, which was locked on the day of the search, should have been opened and checked.
As investigators would later learn, Trump allegedly had the closet's lock changed while his attorney was in Mar-a-Lago's basement, searching for classified documents in a storage room that he was told would have all such documents. Trump's alleged efforts to conceal classified documents from both the FBI and his own attorney are a key part of Smith's indictment against Trump in Florida.
Jordan Strauss, a former federal prosecutor and former national security official in the Justice Department, called the FBI's alleged failure to search the closet "a bit astonishing."
"You're searching a former president's house. You [should] get it right the first time," Strauss told ABC News.
In addition to the closet, the FBI also didn't search what authorities have called a "hidden room" connected to Trump's bedroom, sources said.
Smith's investigators were later told that, in the days right after the search, some of Trump's employees heard that the FBI had missed at least one room at Mar-a-Lago, the sources said.
According to a senior FBI official, agents focused on areas they believed might have government documents.
"Based on information gathered throughout the course of the investigation, areas were identified and searched pursuant to the search warrant," the official told ABC News.
Within a few months of the FBI's search, federal prosecutors in the Justice Department pushed Trump's legal team to ensure that no classified documents remained at any of Trump's properties, but it's unclear if those prosecutors or any Trump lawyers even knew about the unexamined spaces then.
It's also unclear if Trump ever kept any classified documents in either of those spaces, or whether Smith's team ever considered seeking another warrant to search Mar-a-Lago again.
In their questioning of witnesses, Smith's team seemed to focus more on the missed spaces in the three months before first indicting Trump in the case, sources said.
Reached by ABC News, a spokesperson for the Trump campaign criticized President Joe Biden and the news media, saying the investigations into Trump are "just desperate attempts at election interference ... to stop the presumptive Republican nominee for President."
Strauss, who served in the Justice Department from 2005 to 2016, said he was particularly surprised to hear about the FBI's alleged inaction considering how "exceptionally thorough" he said they usually are and how meticulously they planned for the Mar-a-Lago search ahead of time.
Testifying before Congress last year, FBI Director Chris Wray noted that agents conducting the search even wore casual clothes to Mar-a-Lago -- rather than the more common "raid jackets" -- so they wouldn't draw too much attention.
Wray assured lawmakers that in such "sensitive" investigations, "Our folks take great pains to be rigorous [and] professional."
But when agents reached the locked closet near the front of Trump's residence, they couldn't locate a key for it and were told the space behind the door -- an old stairwell turned into a closet with shelves -- went nowhere, so they decided not to break it open, sources said.
Sources also told ABC News that FBI agents didn't do more in part because they felt like they had been at Mar-a-Lago long enough. But the senior FBI official disputed that, saying, "Discussions took place that day about additional areas of the property and it was determined that actions already taken met the parameters of the search warrant."
"[The FBI] is almost notorious for their relentlessness and follow-through," Strauss said.
At the time, the FBI didn't know the lock change -- at least in their view -- could have been potentially significant, sources said.
According to the indictment against Trump, after Trump received a federal subpoena demanding the return of all classified documents at Mar-a-Lago, his attorney -- identified to ABC News as Evan Corcoran -- was told to look for any responsive documents in boxes stacked inside a basement storage room.
But in the days before Corcoran arrived at Mar-a-Lago on June 2, 2022, Trump aide Walt Nauta -- acting "at Trump's direction" -- moved more than 30 boxes from the storage room to Trump's residence, so the attorney never even saw many of Trump's boxes, according to the indictment.
Corcoran found 38 classified documents in the storage room and gave them to the FBI, but Trump ensured that "many documents responsive to the [subpoena] could not be found," the indictment alleges.
Through their investigation, Smith's team learned that while Corcoran was still in the storage room, Trump asked a longtime Mar-a-Lago employee to change the lock on the closet, sources said. For years, the lock on the closet was managed by the Secret Service, but on June 2, 2022, Trump had it changed and wanted the key, the sources said.
One former maintenance worker described Trump's request as unusual, according to the sources.
Unlike the locked closet, the FBI didn't even know the so-called "hidden room" existed until after they left Mar-a-Lago, sources said.
MORE: Trump visits secure facility to view evidence in classified documents case: SourcesThough agents searched Trump's bedroom, a small door in one of the walls was concealed behind a large dresser and a big TV, sources said. The space behind the wall was the "hidden room," which maintenance workers sporadically entered to access cables running through it, sources said.
Strauss said it's not uncommon for agents executing search warrants to miss some things, especially when they're searching expansive properties.
Nevertheless, the fact that witnesses were saying the FBI missed a "hidden room" within Trump's bedroom caught the attention of Smith's team, according to sources.
A federal judge had signed off on the search of Mar-a-Lago, approving the FBI's plan to search Trump's office and "all storage rooms and any other rooms or locations where boxes or records may be stored."
During their search, they allegedly found 27 classified documents in Trump's office and 75 more in the basement storage room, where Corcoran had searched two months earlier and found a smaller set of other apparently classified documents, according to the indictment against Trump.
The FBI did not find classified documents in any ballroom, bathroom, or in Trump's bedroom, where he allegedly stored classified documents at times over the year-and-a-half after leaving the White House.
During the summer of the FBI search, Trump was primarily living at his property in Bedminster, New Jersey. The FBI didn't search that property -- it only searched Mar-a-Lago.
MORE: Trump allegedly discussed US nuclear subs with foreign national after leaving White House: SourcesAs ABC News previously reported, within months of the FBI search, the Justice Department suspected Trump was still holding classified documents somewhere, so -- under pressure from the department -- one of Trump's attorneys conducted another search of Mar-a-Lago and other properties, and he found a handful of more classified documents.
In his testimony to Congress last year, Wray said that, under "specific rules," there are only certain locations that can securely store classified information, "and in my experience, ballrooms, bathrooms and bedrooms are not" among them.
"Our folks in this case have proceeded honorably and in strict compliance with our policies, our rules, and our best practices," Wray added.
Trump has denied any wrongdoing, insisting he did not break the law by holding onto the documents later seized by the FBI. He has pleaded not guilty to the charges against him.
Nauta, the aide who allegedly helped move Trump's boxes, and Mar-a-Lago's property manager, Carlos De Oliveira, have also been charged for their alleged roles in Trump's conspiracy. Both have pleaded not guilty.