Donald Trump’s lawyers turned over an empty manilla folder marked “Classified Evening Briefing” after the US justice department issued a subpoena for its surrender once prosecutors became aware that it was located inside the private quarters of the former president’s Mar-a-Lago resort, two sources familiar with the matter said.

The previously unreported subpoena was issued last month, the sources said, as the recently appointed special counsel escalates the inquiry into Trump’s possible unauthorized retention of national security materials and obstruction of justice.

The folder was seen in Trump’s residence by a team of investigators he hired to search his properties last year for any remaining documents marked as classified. The team transparently included the observation in an inventory of Mar-a-Lago and Trump properties in Florida, New Jersey and New York.

Weeks after the report was sent to the justice department, the sources said, federal prosecutors subpoenaed the folder. The folder is understood to have not been initially returned because the lawyers thought “Classified Evening Briefing” did not make it classified, nor is it a formal classification marking.

The backstory the justice department was told about the folder was that Trump would sometimes ask to keep the envelopes, featuring only the “Classified Evening Briefings” in red lettering, as keepsakes after briefings were delivered, one of the sources said.

Around the same time that Trump’s lawyers turned over the empty folder – earlier reported by CNN – they also returned in December a box of presidential schedules at Mar-a-Lago of which a couple were marked as classified, and in January, a laptop on to which the contents of the box had been scanned last year by a junior aide.

The mishandling of those materials appears to have been inadvertent – in which case, the justice department would be unlikely to include them in the criminal investigation, which has been far more focused on the documents that the FBI seized from Mar-a-Lago last summer.

But the contentious saga reflects the deteriorating relationship between federal prosecutors who have become frustrated at Trump’s resistance towards the inquiry and his lawyers who have complained that the justice department has been unnecessarily heavy-handed at every turn.

A spokesperson for the special counsel’s office declined to comment.

Late last year, Trump hired a team of two private contractors with security clearances to search his properties after the department told his lawyers that they suspected the former president was still in possession of classified-marked documents even after the FBI search in August.

The contractors found and immediately returned two documents, both marked as classified at the “SECRET” level, from boxes that appeared to have been unopened since they were shipped from the White House at the end of the Trump administration, the Guardian previously reported.

Then, at Mar-a-Lago in December, the contractors found a box that mainly contained presidential schedules, in which they found a couple of classified-marked documents to also be present and alerted the legal team to return the materials to the justice department, the sources said.

The exact nature of the classified-marked documents remains unclear, but a person with knowledge of the search likened their sensitivity to schedules for presidential movements – for instance, presidential travel to Afghanistan – that are considered sensitive until they have taken place.

After the Trump legal team turned over the box of schedules, the sources said, they learned that a junior Trump aide – employed by Trump’s Save America political action committee who acted as an assistant in Trump’s political “45 Office” – last year scanned and uploaded the contents of the box to a laptop.

The junior Trump aide, according to what one of the sources said, was apparently instructed to upload the documents by top Trump aide Molly Michael to create a repository of what Trump was doing while in office and was apparently careless in scanning them on to her work laptop.

When the Trump legal team told the justice department about the uploads, federal prosecutors demanded the laptop and its password, warning that they would otherwise move to obtain a grand jury subpoena summoning the junior aide to Washington to grant them access to the computer.

To avoid a subpoena, the Trump legal team agreed to turn over the laptop in its entirety last month, though they did not allow federal prosecutors to collect it from Mar-a-Lago.

ABC News earlier reported the handover.

“This is nothing more than a politically-motivated witch-hunt against President Trump,” a spokesman for Trump said in a statement. “The weaponized Department of Injustice has shown no regard for common decency and key rules that govern the legal system.”

It was around the same time in January as the justice department retrieved the laptop that federal prosecutors in the office of the Trump special counsel Jack Smith issued a grand jury subpoena for the manilla folder marked “Classified Evening Briefing” observed in Trump’s Mar-a-Lago private quarters.