Federal prosecutors investigating former President Donald Trump’s handling of classified documents have obtained the confidential cooperation of a person who has worked for him at Mar-a-Lago, part of an intensifying effort to determine whether Trump ordered boxes containing sensitive material moved out of a storage room there as the government sought to recover it last year, multiple people familiar with the inquiry said.

Through a wave of new subpoenas and grand jury testimony, the Justice Department is moving aggressively to develop a fuller picture of how the documents Trump took with him from the White House were stored, who had access to them, how the security camera system at Mar-a-Lago works and what Trump told aides and his lawyers about what material he had and where it was, the people said.

At the heart of the inquiry is whether Trump sought to hide some documents after the Justice Department issued a subpoena last May demanding their return.

The existence of an insider witness, whose identity has not been disclosed, could be a significant step in the investigation, which is being overseen by Jack Smith, the special counsel appointed by Attorney General Merrick Garland. The witness is said to have provided investigators with a picture of the storage room where the material had been held. Little else is known about what prosecutors might have learned from the witness or when the witness first began to provide information to the prosecutors.

But prosecutors appear to be trying to fill in some gaps in their knowledge about the movement of the boxes, created in part by their handling of another potentially key witness, Trump’s valet, Walt Nauta. Prosecutors believe Nauta has failed to provide them with a full and accurate account of his role in any movement of boxes containing classified documents.

In the past few weeks, at least four more Mar-a-Lago employees have been subpoenaed, along with another person who had visibility into Trump’s thinking when he first returned material to the National Archives, according to people briefed on the matter. Two people said that nearly everyone who works at Mar-a-Lago has been subpoenaed, and that some who serve in fairly obscure jobs have been asked back by investigators.

Prosecutors have also issued several subpoenas to Trump’s company, the Trump Organization, seeking additional surveillance footage from Mar-a-Lago, his residence and private club in Florida, people with knowledge of the matter said. While the footage could shed light on the movement of the boxes, prosecutors have questioned a number of witnesses about gaps in the footage, one of the people said.

But hoping to understand why some of the footage from the storage camera appears to be missing or unavailable — and whether that was a technological issue or something else — the prosecutors subpoenaed the software company that handles all of the surveillance footage for the Trump Organization, including at Mar-a-Lago.

A spokesperson for Trump called the case “a targeted, politically motivated witch hunt” that is “concocted to meddle in an election and prevent the American people from returning him to the White House.” The spokesperson accused Smith’s office of harassing “anyone who has worked for President Trump” and of now using the inquiry to target Trump’s business.

Investigators have been piecing together Trump’s handling of government documents for months, seeking information not just about his habits after leaving the White House but also about his practices as president. Among the information they have gathered in interviews concerned his habit of flushing material down toilets, according to a person familiar with the matter.

Another related line of inquiry for Smith’s team is whether Trump misled one of his lawyers, M. Evan Corcoran, about the movement of classified documents around Mar-a-Lago. In June, Corcoran helped draft a sworn statement, signed by another lawyer, saying that a “diligent search” was conducted of the boxes and that any classified documents were turned over to the Justice Department.

Although Corcoran testified that Trump did not personally convey that false information, his testimony hardly absolved the former president, the people with knowledge of the matter said. Corcoran also recounted to the grand jury how Trump did not tell his lawyers of any other locations where the documents were stored, which may have effectively misled the legal team.

Prosecutors working under Smith have developed what multiple people familiar with the investigation say is a wealth of testimony and evidence about Trump’s behavior during the lengthy period when the National Archives and the Justice Department sought to retrieve presidential materials from the former president.

After months of requests, Trump in January 2022 turned over to the archives 15 boxes of material he had taken from the White House. Those boxes turned out to contain reams of classified material, prompting a Justice Department investigation and a subpoena in May of last year demanding the return of any further documents in Trump’s possession.

The Justice Department investigation has returned repeatedly in recent weeks to a crucial question: Did Trump instruct Nauta, or anyone else, to move boxes out of the storage room before the lawyers conducted the “diligent search” of Mar-a-Lago and said no classified records remained at the property?

Last fall, prosecutors faced a critical decision after investigators felt Nauta had misled them. To gain Nauta’s cooperation, prosecutors could have used a carrot and negotiated with his lawyers, explaining that Nauta would face no legal consequences as long as he gave a thorough version of what had gone on behind closed doors at the property.

Or the prosecutors could have used a stick and wielded the specter of criminal charges to push — or even frighten — Nauta into telling them what they wanted to know.

The prosecutors went with the stick, telling Nauta’s lawyers that he was under investigation and they were considering charging him with a crime.

The move backfired, as Nauta’s lawyers more or less cut off communication with the government. Some investigators believed that top prosecutors, including Jay Bratt, the head of the counterespionage section of the national security division at the Justice Department, had mishandled Nauta and cut off a chance to win his voluntary cooperation.