The latest twist in the inquiry into former President Donald J. Trump’s handling of classified documents is the surprise revelation that a previously unknown federal grand jury in Florida has recently started hearing testimony in the case.

The grand jury in Florida is separate from the one that has been sitting for months in Washington and has been the center of activity for prosecutors as they investigate whether Mr. Trump mishandled classified documents after leaving office or obstructed efforts to retrieve them. Among those who have appeared before the Washington grand jury in the past few months or have been subpoenaed by it, people familiar with the investigation said, are more than 20 members of Mr. Trump’s Secret Service security detail.

But there are indications that the Washington grand jury — located in the city’s federal courthouse — may have stopped hearing witness testimony in recent weeks, according to three people familiar with its workings.

As for the Florida grand jury, which began hearing evidence last month, only a handful of witnesses have testified to it or are scheduled to appear before it, according to the people familiar with its workings. At least one witness has already testified there, and another is set to testify on Wednesday.

It is an open question why prosecutors impaneled the Florida grand jury — which is sitting in Federal District Court in Miami — and whether it is now the only one hearing testimony. This uncertainty, which is largely due to the secret nature of grand juries, serves to underscore how much about the management of the documents case by the special counsel Jack Smith remains out of public view.

“We know a tiny fraction of what agents and prosecutors know, and so it is hazardous, if not impossible, to figure out the government’s strategy from afar,” said Chuck Rosenberg, a former U.S. attorney and F.B.I. official. “It is like the guy berating an umpire for missed calls from the cheap seats.”

But even though much is shrouded in mystery, legal experts and people familiar with the inquiry suggested that there could be a number of reasons Mr. Smith may have chosen to use a grand jury in Florida for at least some elements of the case. His decision could have significant effects on how the inquiry plays out.

In simple terms, the people familiar with the matter said, if both grand juries are in operation, it suggests that prosecutors are considering bringing charges in both Washington and Florida. It is possible that Mr. Trump could be charged in one jurisdiction while other people involved in the case are charged in the other.

But if only the Florida grand jury is currently hearing testimony, it suggests two possibilities.

One is that the investigation in Washington is largely complete and that prosecutors are now poised to make a decision about bringing charges there while still weighing other potential indictments in Florida.

The other is that Mr. Smith has decided that Florida is the proper venue for any charges he might bring in the case and has moved the entire grand jury proceeding there, they said.

(Another possibility is that the Florida grand jury is merely hearing evidence as a convenience to local witnesses — though that option would seem less likely since more than one witness has appeared before it.)

It would not be so unusual if Mr. Smith’s team opened the documents investigation in Washington and then chose to move it to Florida because of legal issues related to venue, said Brandon L. Van Grack, a former federal prosecutor who worked on cases involving national security and classified material.

“It’s common in situations involving classified information when prosecutors are uncertain of venue to ground an investigation in Washington, Virginia or Maryland,” Mr. Van Grack said. “The point is just because it starts there, doesn’t mean it has to end there. You don’t know what your potential venue hooks are until you’ve completed a thorough investigation.”

Mr. Van Grack said it would be relatively easy to move a grand jury inquiry from Washington to Miami if needed. Prosecutors would simply have to read the early grand jury transcripts to the new grand jurors or have federal agents offer them a summary of the most important points.

If Mr. Smith is considering bringing charges in both Washington and Miami, it is possible that the latter might involve potential targets who live and work in Florida. Investigators, for example, have been scrutinizing the roles of two employees of Mr. Trump at Mar-a-Lago, his private club and residence, for their roles in the storage and security of classified documents there.

Even if the Washington grand jury is now on a hiatus, it is possible that it could reconvene and vote to charge Mr. Trump. If it does not, however, and only the Florida grand jury remains in operation, that would suggest that Mr. Smith’s office had concluded that a case against Mr. Trump or his aides should be based in Florida alone, the people familiar with the matter said.

There would be reasons for that last decision, said Timothy Parlatore, a lawyer who resigned last month from Mr. Trump’s legal team. Many of the central events of the documents inquiry took place when Mr. Trump was living in Florida, he said.

Mr. Trump resided at Mar-a-Lago when he and his lawyers first began negotiating the return of government records to the National Archives in late 2021. And the initial trove of classified documents that the archives discovered was in a batch of 15 boxes of records that Mr. Trump sent up to Washington from Florida.

Florida is where Mr. Trump was living when the Justice Department issued its subpoena last May for the return of all classified documents in the possession of his presidential office. And when prosecutors from Washington sought a meeting with Mr. Trump’s lawyers to enforce that subpoena and collect any relevant material, it took place at Mar-a-Lago.

After prosecutors came to believe that Mr. Trump was still holding on to classified material even after the subpoena, they sent the F.B.I. to search Mar-a-Lago. Agents hauled away another 100 or so classified documents that were discovered at the Florida compound in violation of the subpoena.

But prosecutors could still seek to establish venue in Washington for charges against Mr. Trump, especially given that moving a potential case to Miami would not be without risk for Mr. Smith and his team.

A Florida jury might prove to be more sympathetic to Mr. Trump than a Washington jury. And the judges in the Southern District of Florida — among them Aileen M. Cannon, who made an unusual decision to pause the investigation in its early stages to have an outside arbiter review the documents seized at Mar-a-Lago — might be more inclined to rule in Mr. Trump’s favor than those in Washington.

Jonathan Swan contributed reporting.