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 in Washington, 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.