Former President Donald J. Trump said on Tuesday that he had received a target letter from the special counsel investigating the Jan. 6 riot, the second time he has been notified that he is a target in a federal investigation.

A target letter is an official piece of correspondence from the Justice Department informing someone that he or she is being investigated. It does not formally charge a person, but indicates that an investigation is nearing its end and that the department is actively considering charging them.

Typically, such a letter provides the recipient the opportunity to testify before a grand jury but does not obligate them to appear. Mitchell Epner, a former assistant United States Attorney for the district of New Jersey, said targets of investigations rarely avail themselves of that opportunity.

“I would be very surprised if Mr. Trump were to testify before the grand jury,” he said.

Regardless of whether Mr. Trump testifies, Mr. Epner said that he expected the Justice Department to present a potential indictment to the grand jury soon and that a charging decision would be made quickly. Of a minimum 16 grand jurors, at least 12 votes are needed to indict Mr. Trump.

“It could be as little as days but more typically weeks, sometimes months,” Mr. Epner said, adding that a decision would almost certainly be made this year.

Mr. Trump has not released his target letter. But a sample letter from a Justice Department manual for prosecutors describes how one would read.

Among other things, the sample letter says: “You are advised that you are a target of the grand jury’s investigation. You may refuse to answer any question if a truthful answer to the question would tend to incriminate you. Anything that you do or say may be used against you in a subsequent legal proceeding. If you have retained counsel, who represents you personally, the grand jury will permit you a reasonable opportunity to step outside the grand jury room and confer with counsel if you desire.”

Charlie Savage contributed reporting.