The document, titled “1776 Returns,” was cited by prosecutors last week in charging the far-right leader, Enrique Tarrio, the former head of the Proud Boys extremist group, with conspiracy. The indictment of Mr. Tarrio described the document in general terms, but the people familiar with it added substantial new details about the scope and complexity of the plan it set out for directing an effort to occupy six House and Senate office buildings and the Supreme Court last Jan. 6.

The document does not specifically mention an attack on the Capitol building itself. But in targeting high-profile government buildings in the immediate area and in the detailed timeline it set out, the plan closely resembles what actually unfolded when the Capitol was stormed by a pro-Trump mob intent on disrupting congressional certification of President Biden’s Electoral College victory.

Many questions remain about the document, including who wrote it and how it made its way to Mr. Tarrio, according to prosecutors, on Dec. 30, 2020, as President Donald J. Trump was engaged in a series of overlapping schemes to keep himself in power. The people familiar with the document said other evidence the government has gathered suggests that it may have been provided to Mr. Tarrio by one of his girlfriends at the time.