Washington: Special counsel Jack Smith has subpoenaed former US national security adviser Robert O'Brien as part of his investigations into classified documents discovered at the Mar-a-Lago residence of former President Donald Trump. Smith is also looking into efforts to invalidate the results of the 2020 election. In refusing to provide some of the information that prosecutors are requesting from him, O'Brien has been citing executive privilege, according to the CNN report.

An inquiry for comment was not immediately answered by Smith's office. According to the report, which also cited sources, Justice Department attorneys recently questioned Chad Wolf, Trump's former acting secretary of the Department of Homeland Security, as part of the special counsel's ongoing investigation into potential interference with the 2020 election.

Shortly after Trump declared he would run for president again in 2024 on the Republican ticket, US Attorney General Merrick Garland appointed Smith as special counsel to lead investigations into him in November.

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The initial investigation concerns how Trump handled highly confidential documents that he kept at his Florida resort after leaving the White House in January 2021.

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The second investigation focuses on attempts to void the outcome of the 2020 presidential election, including a scheme to present fictitious slates of electors to prevent Congress from recognising Democrat Joe Biden as the winner.

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Many former top Trump administration officials have testified before grand juries in Washington in recent months for both investigations. Robert Hur, a different special counsel, was appointed by Garland last month to look into the improper storage of classified documents at President Biden's residence and former workplace.