People familiar with the investigation said on Wednesday that federal prosecutors have significantly broadened their Jan. 6 investigation to look into the possible culpability of a wide range of figures involved in former President Donald J. Trump’s efforts to overturn the results of the 2020 election.

Other government officials may have been involved in Mr. Trump’s attempts to obstruct the certification of President Biden’s Electoral College victory, as well as the push by some Trump allies to promote fake elector slates, they said.

Prosecutors are also interested in the preparations for the rallies that preceded the assault on the Capitol, such as the rally on the Ellipse on Jan. 6, just before a pro-Trump mob stormed the building.

The federal investigation initially concentrated on the rioters who had broken into the Capitol, resulting in over 700 arrests. The Justice Department, on the other hand, appears to have entered a new phase, in which it is looking for information on people who are more closely linked to Mr. Trump. This development comes as Attorney General Merrick B. Garland faces increasing political pressure to act more aggressively in the case.

According to a source familiar with the situation, a grand jury in Washington is looking into the rallies that preceded the Capitol storming.

It also wanted to know if any members of the executive or legislative branches were involved in the “planning or execution of any rally or any attempt to obstruct, influence, impede, or delay” the certification of the 2020 election.

It also inquired about Trump supporters’ efforts to put forward alternate slates of electors, as Mr. Trump and his allies sought to overturn Congress’ certification of the Electoral College outcome on Jan. 6.

At least one person involved in the logistics of the Jan. 6 rally had been asked to appear, according to another person briefed on the grand jury investigation.

Prosecutors have been gathering evidence documenting how defendants have used statements from Mr. Trump to explain why they stormed the Capitol on Jan. 6. In some cases, prosecutors have cited a tweet from Mr. Trump weeks before Jan. 6 exhorting his followers to come to Washington, a call that fueled extremist groups in particular.

A separate investigation by the House select committee on the Capitol riot is gathering evidence about Mr. Trump’s efforts to maintain power and weighing the possibility of referring Mr. Trump to the Justice Department for criminal prosecution.

In a civil case involving the House committee, a federal judge in California concluded on Monday that Mr. Trump likely engaged in criminal conduct, including obstructing the work of Congress and conspiring to defraud the United States.

Mr. Garland has given no indication whether the Justice Department will pursue Mr. Trump, saying only that the department will follow the facts wherever they lead. The Justice Department previously stated that it was investigating elector slates that incorrectly declared Mr. Trump the winner in seven swing states won by Mr. Biden.

Despite the fact that election officials in the seven contested states sent official lists of electors who voted for Mr. Biden to the Electoral College, the fake slates claimed Mr. Trump had won in an apparent attempt to sway the election result.

Legislators, state officials, and the House committee investigating the riot on Jan. 6 had asked the Justice Department to investigate the role of the fake electors and the documents they submitted to the National Archives on Dec. 14, 2020. The grand jury subpoenas indicate that prosecutors are trying to figure out if submitting the documents to a federal agency was illegal.

Prosecutors charged Stewart Rhodes, the founder and leader of the Oath Keepers militia, with seditious conspiracy in January, accusing him of plotting to violently disrupt Congress’ work.