Federal investigators are questioning witnesses about former President Trump's conduct and examining his conversations with aides concerning the 2020 election as part of their criminal investigation, the Washington Post first reported Tuesday.

Why it matters: This would mark a major escalation in the Justice Department's investigation into efforts to overturn the 2020 presidential election. The reports came hours after Attorney General Merrick Garland declined to rule out prosecuting Trump.

• Federal investigators would "pursue justice without fear or favor" over events surrounding the U.S. Capitol riot or attempts "to interfere with the lawful transfer of power from one administration to another," Garland told NBC News on Tuesday.

For the record: There's no evidence that Trump is himself an investigation target, but "prosecutors are asking questions related to him and his aides," per CBS News, which also reported on the Justice Department investigation.

Driving the news: Prosecutors have questioned witnesses testifying before a grand jury in recent days about conversations with Trump and his close advisers about "fake electors," according to multiple reports.

• "People familiar with the probe said investigators are examining the former president’s conversations and have seized phone records of top aides," per WashPost.

Zoom in: Two witnesses who were senior aides to former Vice President Mike Pence in the Trump administration, Marc Short and lawyer Greg Jacob, were among those questioned, several outlets reported.

• The focus of the questions on Trump concerned matters such as a plan to thwart the congressional certification of President Biden's Electoral College win on Jan. 6, 2021, according to the New York Times.
• Short, Pence's former chief of staff, confirmed Monday that he had testified before the grand jury, but declined to comment further.

Meanwhile, the Jan. 6 House select committee has a laser focus on Trump and anything he may have done to encourage or prolong the Capitol attack, assembling piles of transcripts and other evidence that could be used in federal prosecutions, Axios' Mike Allen notes.

Worth noting: Trump has repeatedly made false claims about the 2020 election result, including at a rally before the Capitol insurrection on Jan. 6, 2021, and also at an event on Tuesday night. But he has always denied any criminal wrongdoing.

• Representatives for Trump and the DOJ could not immediately be reached for comment.

Go deeper... Seizure of ex-Trump attorney's phone in Jan. 6 probe lawful: Judge

Editor's note: This article has been updated with new details throughout.