Stephen Bannon, President Donald Trump’s former chief strategist, was subpoenaed last week by the special counsel, Robert Mueller, to testify before a grand jury as part of the investigation into possible links between Mr Trump’s associates and Russia, according to a person with direct knowledge of the matter.

The move marked the first time Mr Mueller is known to have used a grand jury subpoena to seek information from a member of Mr Trump’s inner circle.

The special counsel’s office has used subpoenas before to seek information on Mr Trump’s associates and their possible ties to Russia or other foreign governments.

The subpoena could be a negotiating tactic. Mr Mueller is likely to allow Mr Bannon to forgo the grand jury appearance if he agrees to instead be questioned by investigators in the less formal setting of the special counsel’s offices about ties between Mr Trump’s associates and Russia and about the president’s conduct in office, according to the person, who would not be named discussing the case.

But it was not clear why Mr Mueller treated Mr Bannon differently than the dozen administration officials who were interviewed in the final months of last year and were never served with a subpoena.

The subpoena is a sign that Mr Bannon is not personally the focus of the investigation. Justice Department rules allow prosecutors to subpoena to the targets of investigations only in rare circumstances.

On Tuesday, Mr Bannon testified behind closed doors before the House Intelligence Committee, which is also investigating Russia’s meddling in the 2016 election and ties between the Trump campaign and Russia.

Mr Bannon did not address reporters before entering the proceeding on Tuesday, and a spokesman for Mr Mueller and a senior White House lawyer did not respond to messages seeking comment.

Mr Mueller issued the subpoena after Mr Bannon was quoted in a new book criticising Mr Trump, saying that Donald Trump Jr’s 2016 meeting with Russians was “treasonous” and predicting that the special counsel investigation would ultimately center on money laundering.

After excerpts from the book, Fire and Fury: Inside the Trump White House, were published this month, Mr Trump derided Mr Bannon publicly and threatened to sue him for defamation. Mr Bannon was soon ousted as the executive chairman of the hard-right website Breitbart News.

Wolff’s Fire and Fury incensed Trump and his team but delighted booksellers. Photo: Getty

Some legal experts said the subpoena could be a sign that the investigation was intensifying, while others said it may simply have been a negotiating tactic to persuade Mr Bannon to cooperate with the investigation.

The experts also said it could be a signal to Mr Bannon, who has tried to publicly patch up his falling-out with the president, that despite Mr Trump’s legal threats, Mr Bannon must be completely forthcoming with investigators.

Prosecutors generally prefer to interview witnesses before a grand jury when they believe they have information that the witnesses do not know or when they think they might catch the witnesses in a lie.

It is much easier for a witness to stop the questioning or sidestep questions in an interview than during grand jury testimony, which is transcribed, and witnesses are required to answer every question.

“By forcing someone to testify through a subpoena, you are providing the witness with cover because they can say, ‘I had no choice — I had to go in and testify about everything I knew,’” Solomon Wisenberg, a prosecutor for the independent counsel that investigated Bill Clinton when he was president, said.

Mr Bannon has limited firsthand knowledge about two key issues within Mr Mueller’s purview — the president’s firing of James Comey as FBI director, a decision made without Mr Bannon present, and the drafting of a misleading statement about the subject of the June 2016 meeting with Russians, in which they promised damaging information about Hillary Clinton.

But even Mr Bannon’s secondhand knowledge could be used to draw a contrast with statements from people with firsthand knowledge whom Mr Mueller has already interviewed.

And Mr Bannon was directly involved in a number of other major moments, including the decision-making around the firing of Michael Flynn, the president’s first national security adviser, who was dismissed after he lied to Vice President Mike Pence about phone calls with the Russian ambassador during the presidential transition.

Mr Bannon also helped run the transition after Chris Christie, the outgoing governor of New Jersey, was fired as head of that team.

And Mr Bannon was the chief executive of the Trump campaign in October 2016 when WikiLeaks began releasing thousands of stolen personal emails from the hacked account of Mrs. Clinton’s campaign chairman, John Podesta.

 – The New York Times