His intermediaries repeatedly delivered a message that Mr. Mueller, then the special counsel, conveyed last month in a rare public appearance: A prosecutor speaks through his indictments and the written word rather than the public spectacle of a congressional hearing. Mr. Mueller was so averse to being pulled into the political arena that he never spoke directly with lawmakers or their aides, according to a senior congressional official involved in the talks and others briefed on them.
His reticence mattered little in the end. Democrats were insistent that he had a responsibility to testify, though they agreed to combine questioning from two panels on one day. The protracted negotiations came to an abrupt stop late on Tuesday night when representatives for Mr. Mueller agreed that he would show up if the House Judiciary and Intelligence Committees issued subpoenas for an appearance on July 17.