An aide to former president Donald Trump was to appear before a federal grand jury in Washington on Thursday, people familiar with the matter said, as the Justice Department moves toward deciding whether to criminally charge Trump in connection with his efforts to subvert the results of the 2020 presidential election.

William Russell served in the White House as a special assistant and the deputy director of presidential advance operations and continued to work as a personal aide for Trump after Trump left office in January, 2021.

Russell was asked to appear Thursday after making prior grand jury appearances, according to the people familiar with the matter, who like others interviewed for this article spoke on the condition of anonymity because of grand jury secrecy rules.

A Justice Department investigation led by special counsel Jack Smith has been examining efforts by Trump and his allies to overturn Joe Biden's victory in the 2020 presidential election including the events that led up to the Jan. 6, 2021, riot at the U.S. Capitol.

Investigators have looked into Trump's knowledge of efforts by lawyers and advisers to substitute Trump allies for certified electors in some states Joe Biden had won; what Trump told aides regarding an effort to use those electors to overturn the election; and his pressure campaign on then-Vice President Mike Pence to assist him in the effort. Prosecutors also have sought evidence about ads and email messages related to efforts to raise money off false claims of election fraud, as well as the decision by supposed Republican electors in some states won by Biden to send signed statements purporting to affirm Trump as the victor in those states.

Dozens of witnesses have been questioned, some of them multiple times.

Separate grand juries have heard evidence in Washington and Miami on a different investigation Smith is leading, which involves Trump's possession of classified documents after leaving office and his alleged attempts to obstruct government efforts to retrieve them. Trump and his longtime valet, Waltine "Walt" Nauta, were charged in a 38-count indictment in that case last month and have pleaded not guilty.

The Post could not confirm which subject Russell was being asked to discuss Thursday. A spokesperson for Smith declined to comment.

Trump posted on his social media platform Tuesday morning that he had received a letter from the Justice Department saying he was a target of the investigation and inviting him to submit to questioning under oath before the grand jury as well, a standard offer when someone is likely to be charged with a crime.

He said he was given four days to appear. An adviser to the former president said Wednesday not to expect Trump to accept the grand jury invitation, although no final decision had been made.

Three people familiar with the matter said the letter cited multiple criminal statutes that prosecutors are examining.

In addition to Russell, Smith's team recently contacted and is the process of scheduling a grand jury appearance by Bernard Kerik, a former New York City police commissioner who worked with Trump attorney Rudolph Giuliani in the effort to overturn the election results, according to two people familiar with the matter. Kerik, who with Giuliani tried to unearth evidence of election fraud, voluntarily appeared for an interview before the House select committee that investigated the Jan. 6, 2021, attack on the U.S. Capitol. However, he declined to share some materials sought by the committee, declaring that they were privileged.

No one other than Trump so far has confirmed receipt of a target letter in the election interference probe, although that does not mean that others have not received such notifications or will not in coming days.

Ongoing witness interviews do not necessarily mean prosecutors are planning new charges; special counsel investigations also are authorized to produce public reports on the basis of the evidence they compile. But an initial indictment - of Trump or anyone else - would not rule out superseding charges filed later, potentially bringing more counts or additional defendants.

Smith's team sent a letter to Trump's lawyers in May informing them that the former president was a target of the classified-documents investigation. Three weeks later, Trump and Nauta were charged.

A federal judge in South Florida is weighing when to schedule the trial; that decision is complicated by the fact that Trump also faces state charges of falsifying business records in New York and is again running for president.

The district attorney in Fulton County, Ga., also is nearing a charging decision in her investigation of the efforts to reverse the 2020 election results in Georgia.

Trump is the first former president ever charged with crimes on the state or federal level. The New York case is scheduled for trial in March, when the 2024 nominating contests will be underway. Trump also faces civil trials in New York this fall and winter over his company's business practices and allegations that he defamed the writer E. Jean Carroll.

Trump advisers said Thursday that the former president has hired John Lauro - a Florida based lawyer - to buttress his legal team.

The Washington Post's Perry Stein contributed to this report.