WASHINGTON — In the days since Attorney General William P. Barr wrote to Congress describing the still secret report by the special counsel, Robert S. Mueller III, a mystery he introduced remains pressing: Why did Mr. Mueller take no position on whether President Trump obstructed justice?

Mr. Barr spoke briefly by phone on Wednesday with Representative Jerrold Nadler, Democrat of New York and the House Judiciary Committee chairman, who has criticized Mr. Barr’s decision to step into the gap left by Mr. Mueller’s team and clear Mr. Trump himself. Mr. Nadler has called on Mr. Barr to swiftly turn over the entire Mueller report, but he said it was “apparent” after they spoke that the Justice Department would not meet his April 2 deadline and that he would not commit to making it public without redactions.

“We’re not happy about that, to put it mildly,” Mr. Nadler said.

In the meantime, Mr. Barr’s letter offered scant clues. He wrote that Mr. Mueller decided not to make a “traditional prosecutorial judgment” about whether the evidence was sufficient to establish whether Mr. Trump committed a crime but did not explain Mr. Mueller’s reasoning, beyond quoting the special counsel as stating that the question raised “difficult issues” of law and fact.