Richard A. Boucher, a longtime spokesman at the State Department who was the calming, analytical voice of U.S. diplomacy during the attacks of Sept. 11, 2001, and the ensuing military response — and who went on to help frame U.S. policy in Afghanistan, which he acknowledged was a failure — died on Friday at his home in Arlington, Va. He was 73.

The cause was spindle cell carcinoma, his son, Peter Boucher, said.

Mr. Boucher (rhymes with “voucher”) served longer than anyone else as a State Department spokesman, working for six secretaries of state, from George H.W. Bush’s administration to George W. Bush’s — explaining, distilling and defending the views of Republican and Democratic presidents, in a noncombative tone and with a scrupulous lack of interjected personal opinion.

His knowledge and analysis of issues, based on his own foreign service career on multiple continents, was such that officials around the State Department paused their work to tune in to his briefings.

Speaking daily in public without rehearsal “is one of the most difficult jobs” in the foreign service, R. Nicholas Burns, a former under secretary of state for political affairs, said in an interview: “The job of Richard was to advance, explain and defend the administration’s position on any issue. He was able to distill very complex issues into understandable language for the American public.”

Mr. Boucher, who in 2008 was appointed a career ambassador, the highest rank for a diplomat, was traveling with Secretary of State Colin L. Powell in Peru on 9/11.

On the emergency flight home, Mr. Boucher was updating his boss on a department to-do list, he recalled, when Secretary Powell cut him off: “You don’t understand,” he said. “This changes everything.”