The US justice department is intensifying its investigation of Donald Trump’s unauthorized retention of national security materials as it prepares to question the people who searched the former president’s properties at the end of last year and found more documents with classified markings.

The department was given a general explanation from Trump’s lawyers at the time about who conducted the search – a company said to be known to Trump with experience handling classified records cases – when the new documents marked as classified were returned to the government around Thanksgiving last year.

But the department, unsatisfied with that accounting, last week convinced a federal judge in a sealed hearing to force Trump’s lawyers to give the names of the people who retrieved the documents with an intent to question them directly, according to sources familiar with the matter.

The move by prosecutors to ask a federal judge to compel the information marks the latest escalating twist in the criminal investigation into Trump’s potential unauthorized retention of highly sensitive government documents as well as obstruction of justice.

The pattern of prosecutors now seeking judicial intervention at every turn signals an aggressive posture from the special counsel Jack Smith, who is overseeing the investigation after being appointed to insulate the department from accusations of political conflicts with Trump, who is now a 2024 presidential candidate.

The justice department told Trump’s legal team in October that it suspected the former president was still in possession of additional documents with classified markings even after the FBI seized hundreds of sensitive materials when agents searched his Mar-a-Lago property on 8 August.

After initially resisting suggestions to retain an outside firm to search his properties for any classified documents, Trump retained people to search his other properties including Trump Tower in New York, Trump Bedminster golf club in New Jersey, Mar-a-Lago, and a storage unit in Florida.

The search, carried out by a company described as being a known entity to the former president, turned up at the storage unit at least two more documents with classified markings that Trump’s lawyers then hurriedly turned over to prosecutors on the documents case.

But the discovery exasperated the justice department, which in December asked the chief US district court judge for the District of Columbia, Beryl Howell, to hold Trump’s office in contempt of the first subpoena issued to Trump back in May demanding the return of any and all classified documents.

The contempt motion was not immediately granted, the Guardian first reported, and Howell told prosecutors to work out the matter with Trump’s lawyers. Still, the issue lingered, and the department subsequently asked for the names of the people involved in the search.

Trump’s legal team initially demurred and disputed that prosecutors needed to know the names, according to a source familiar with the matter, before they relented and offered to make them available – but only under a protective order because they worried it would leak to the news media.

Frustrated with the back-and-forth with Trump’s lawyers, the department instead filed a motion to compel the names in a sealed hearing last Thursday before Howell, who granted the request after a tense exchange with the former president’s lawyers where she insisted the government needed to know the names.

The order was issued on Thursday, the source said. It was not clear whether the justice department has issued subpoenas for testimony from the people who carried out the search before the grand jury hearing evidence in the Mar-a-Lago documents case. A spokesman for the special counsel’s office declined to comment.

The closed-door proceedings before Howell come as the justice department is understood to be weighing next steps specifically for the documents case, including whether to either immunize witnesses to force their testimony, or threaten charges to pressure them into cooperating with prosecutors.

Prosecutors were already examining last year whether to threaten charges against Trump’s valet, Walt Nauta, who followed the former president from the White House to Mar-a-Lago, after growing skeptical of his initial testimony concerning moving boxes that contained classified materials.

Should the department move to immunize or prosecute Nauta, he would be the latest Trump employee to be hauled before the grand jury in Washington DC after some of Trump’s top aides including Dan Scavino, William Russell and Beau Harrison testified at the start of December.

But it would also probably open a window into how the department intends to proceed with the investigation. Prosecutors are loth to use immunity as it makes future prosecution difficult and former US attorneys say it would be offered only to help construct a case against a bigger target, such as Trump.

The department most recently conferred immunity to the Trump adviser Kash Patel at the start of November over his objections, the Guardian reported, forcing him to testify before the grand jury after he previously asserted his fifth amendment right against self-incrimination in questioning.

The department’s interest in Patel centered on his claims that the documents found at Mar-a-Lago were declassified, how the documents came to end up at the property and how Trump’s aides and lawyers responded to requests for their return, the sources said.

The status of the documents is important because if prosecutors can prove that those seized by the FBI in August were not declassified, it could strengthen a potential obstruction case contending that Trump used the claims as an excuse for why he did not return records that had been subpoenaed.