The US Justice Department is examining Donald Trump’s actions in connection with efforts to overturn the results of the 2020 election, say US media.
Federal prosecutors have reportedly asked witnesses directly about the behaviour of the former US president.
So far, they have chosen not to open a formal criminal investigation into Mr Trump himself.
Rioters stormed the US Capitol on 6 January 2021 in an effort to overturn the president’s election defeat.
No former US president has ever been indicted for criminal conduct.
Mr Trump has publicly praised those who attacked the building, but denies any personal wrongdoing.
The Justice Department already has a criminal investigation into what happened on 6 January. The reports that witnesses are being questioned about Mr Trump’s role does not mean that federal prosecutors will decide to pursue criminal charges against him.
The investigation is separate to the high-profile, televised Congressional hearings that have taken place over the past few weeks on the same subject – which Mr Trump has characterised as a political witch hunt.
According to a report in the Washington Post, federal prosecutors questioned witnesses before a grand jury about their conversations with Mr Trump and his inner circle in the months leading up to the 6 January riot.
The witnesses were reportedly asked about instructions given by Mr Trump in connection to any attempts to prevent President Joe Biden’s election victory from being certified by Congress.
Some of those questioned included senior members of former Vice President Mike Pence’s staff, multiple US outlets report.
Until now the Justice Department has refused to say whether or not it would weigh charges against Mr Trump for any alleged role in trying to overturn his defeat in the 2020 presidential election.
When the department’s top official, Attorney General Merrick Garland, was asked on Tuesday whether he was concerned about indicting a former president – he simply responded that he intends to hold “everyone” accountable.
Federal officials would prosecute anyone “criminally responsible for interfering with the peaceful transfer of power from one administration to another,” Mr Garland told NBC News.
The Justice Department’s probe into what happened on 6 January 2021, he said, is the “most wide-ranging investigation its history”.
Any decision by federal prosecutors to bring charges against a former president – and potential candidate in the 2024 election – would have significant constitutional and political consequences.