The US National Archives confirmed on Friday that officials found classified materials in boxes of documents Donald Trump improperly removed from the White House – and that they had alerted the Department of Justice (DoJ). The disclosure is expected to escalate an investigation by the House oversight committee into whether Trump violated the Presidential Records Act of 1978 by removing and destroying White House documents. In a letter to the committee, David Ferriero, of the National Archives and Records Administration (Nara), said it had “identified items marked as classified national security information in the boxes”. “Because Nara identified classified information in the boxes,” he wrote, “Nara staff has been in communication with the Department of Justice.” The agency also confirmed that deleted tweets from Trump’s personal account and accounts belonging to top White House officials including former press secretary Kayleigh McEnany and senior adviser Peter Navaro have probably been lost forever. Ferriero said: “Some White House staff conducted official business using non-official electronic messaging accounts that were not copied or forwarded into their official electronic messaging accounts.” Nara, he said, was in the process of obtaining those records. The news of potential violations of the Presidential Records Act, which mandates the preservation of White House documents, came after a series of reports that Trump openly flouted the statute. In late January, after protracted negotiations with Trump lawyers, the Archives secured the return of 15 boxes of documents Trump took from the White House to Mar-a-Lago, his post-presidency home in Florida. The boxes included White House documents considered presidential records, as well as items including “love letters” from Kim Jong-un of North Korea, a letter left for Trump by his predecessor as president, Barack Obama, and a model of Air Force One with red-white-and-blue livery Trump chose. But some materials in the 15 boxes were marked as classified, the Archives said. That prompted officials to consult with the DoJ over whether Trump’s actions were potentially unlawful. The chair of the House oversight committee, Carolyn Maloney, noted last week in opening her investigation that “removing or concealing government records is a criminal offense”. Trump must be held accountable, the New York Democrat said. The DoJ has declined to comment on whether it will open a criminal investigation. Experts have said prosecuting violations of the Presidential Records Act is tricky, since it lacks clear enforcement guidelines. The Archives also said in its letter on Friday staff were in the process of inventorying the 15 boxes to determine if other materials from the Trump White House remain missing, work they expected to complete next Friday.