Donald Trump’s lawyers found at least two more documents bearing classification markings inside boxes at a storage unit in Florida when they searched through items that were brought from the White House at the end of his administration, one source familiar with the matter said on Tuesday.

The new discovery could exacerbate the former president’s legal exposure after the FBI seized 103 documents marked classified at his Mar-a-Lago resort in August as part of the justice department’s criminal investigation into the possible unauthorized retention of national security information and obstruction of justice.

The presence of documents marked classified in a second location beyond Mar-a-Lago, earlier reported by the Washington Post, appears to confirm the justice department’s suspicions, communicated to Trump’s lawyers in October, that Trump possessed additional government records.

Trump’s lawyers found the documents after the former president retained an outside firm to search four locations after a federal judge ordered his legal team to conduct a more thorough search to make sure all documents marked classified had been returned to the government.

The outside firm ended up searching a number of Trump’s properties, according to another source, including Trump Tower in New York, Trump Bedminster golf club in New Jersey, the Mar-a-Lago resort and the external storage unit in West Palm Beach, Florida, which has been understood to have been controlled by a federal agency.

According to emails released by the General Services Administration, a government agency that assists in presidential transitions, Trump used a storage facility in West Palm Beach to hold some materials that were packed up from the White House and had been temporarily held in Virginia.

That storage facility was used to hold at least three pallets of boxes that had been packed up by Trump White House staffers and the GSA initially transported to an office space in Virginia before sending them to Florida in September 2021, the emails show.

The contents of the boxes in the pallets do not appear to have ever been catalogued, the second source said. It was not clear whether the storage facility referenced in the emails was the same storage unit where the new documents were found – but it was the place from where Trump’s lawyers sent two dozen boxes to the National Archives earlier this year.

The justice department declined to comment. A Trump spokesman did not immediately respond to a request for comment.

Trump’s lawyers were ordered in recent weeks to conduct a more thorough search of items in the former president’s possession by Beryl Howell, the chief US district court judge for the District of Columbia, in a sealed order issued as part of a closed-door court battle.

The order capped a weeks-long process that started after the justice department expressed concern that Trump still had additional documents marked classified in his possession, potentially at other properties, after the FBI seized thousands of materials at Mar-a-Lago on 8 August.

Trump was served with a grand jury subpoena in May demanding the return of all government records – bearing classification markings or otherwise – in the possession of the “45 Office”, to which his lawyers responded by turning over a double-taped folder containing responsive documents.

The double-taped folder contained documents found by Trump attorney Evan Corcoran in a basement storage room at Mar-a-Lago, the second source told the Guardian, and got another Trump attorney Christina Bobb to sign a caveated attestation certifying compliance with the subpoena.

But in the following months, the justice department developed evidence that other sensitive materials remained at Mar-a-Lago, and the FBI retrieved 103 documents marked classified in Trump’s office and in the basement storage room, according to the unsealed search warrant affidavit.

The justice department then developed suspicions that Trump potentially was in possession of still more government records he should no longer have access to, and eventually asked Howell to intervene and order a second search of Trump’s belongings, the second source said.

Former Florida solicitor general Christopher Kise, who had by then been added to Trump’s legal team, had suggested retaining an outside firm to conduct another search even before the court order, though that idea was initially rejected by some of the more bullish Trump lawyers on the team.

But when the court order necessitated a more thorough search, Trump engaged the outside firm. The FBI is understood to have been invited to observe the search of at least one of the properties, but declined the offer, as is typical for searches not done by law enforcement, the source said.