Key keywords: ex-national security adviser, classified files case, plead guilty, US Department of Justice, unauthorized retention of classified documents, federal criminal charges, national security information, plea agreement WASHINGTON, Oct. 12 (Reuters) - A former U.S. national security adviser who served during the 2017-2021 presidential administration is set to enter a guilty plea in federal court next week in connection with a long-running investigation into his unauthorized retention of hundreds of pages of classified files, three people familiar with the matter told reporters on Thursday. The probe was first launched by the Department of Justice (DOJ) in early 2022 after the National Archives and Records Administration (NARA) reported that the ex-official had failed to turn over dozens of marked classified materials at the end of his White House tenure. Investigators later uncovered more than 300 documents containing sensitive national security information stored in unsecure locations at his private Florida residence, including materials related to ongoing U.S. military operations in the Middle East, undercover foreign intelligence sources, and nuclear program details of a key U.S. ally in the Indo-Pacific region. According to a person with direct knowledge of the plea negotiations, the expected guilty plea is part of a deal with federal prosecutors that will see the former adviser plead guilty to one felony count of willful retention of national defense information, a charge that carries a maximum sentence of 10 years in federal prison. People close to the case said the recommended sentence under the agreement will likely fall between 18 and 24 months, with a possibility of supervised release in lieu of up to half of the prison term, contingent on the former official cooperating with any ongoing related investigations. The ex-adviser had previously maintained that all the documents in his possession were declassified via oral order before he left office, but federal investigators presented irrefutable evidence earlier this year showing that he had repeatedly instructed his personal aides to move classified files out of secure White House facilities and transport them to his private home, even after multiple formal requests from NARA and DOJ officials to return all sensitive government property. The guilty plea will mark one of the most high-profile convictions in a series of DOJ probes into mishandling of classified information by former senior government officials over the past three years. A hearing to formalize the plea is scheduled for next Wednesday at the U.S. District Court for the District of Columbia, where the official is expected to publicly acknowledge his wrongdoing for the first time. Prosecutors are also expected to release a detailed statement of facts outlining the full scope of the classified materials he held, as well as the potential national security risks posed by the years of unsecure storage.