“Mr. Butterfield, are you aware of the installation of any listening devices in the Oval Office of the president?”

It was July 16, 1973, and Alexander Butterfield, a former deputy assistant to President Nixon, paused for five seconds before answering the question posed by a lawyer for the Senate committee investigating Watergate. “I was aware of listening devices,” he finally responded. “Yes, sir.”

The revelation of Nixon’s secret White House tapes proved one of the most dramatic moments in the unfolding Watergate scandal — and cemented Butterfield’s spot in American history. In a forthcoming book based on 46 hours of interviews with Butterfield and thousands of documents that the former Nixon aide took from the White House, Washington Post associate editor Bob Woodward recounts Butterfield’s years in the West Wing and his close working relationship with the president during the Vietnam War and Watergate.

“These are the last pieces of the Nixon puzzle,” Woodward said in an interview.

“The Last of the President’s Men” will be published on Oct. 13 by Simon & Schuster. Woodward conducted the bulk of the interviews with Butterfield in 2014 and 2015, both in Washington and in La Jolla, Calif., where Butterfield resides. The book includes 75 pages worth of documents, “many of them original and not in the presidential archives and libraries,” according to the publisher.

“Butterfield was the consummate gray man, in the background but potent,” Woodward said.

A former Air Force colonel and college friend of Nixon chief of staff H.R. Haldeman, Butterfield began working in the White House in 1969 and, for three years occupied an office adjoining the Oval Office. He was responsible for the flow of memos, briefing papers and correspondence going to the president. According to Woodward’s book, Halderman gave Butterfield the following job description: “I’m really going to be the president’s alter ego. You will be to me what I am to the president.”

At Nixon’s request, Butterfield oversaw the installation of the recording devices. However, “the secret taping system was not put in place until February 1971,” Woodward writes, according to a passage provided by Simon & Schuster. “There are no tapes of the first two years of the Nixon presidency.” Because of the documents he kept, and “by virtue of his proximity to the center of the Nixon universe and his extraordinary memory, Butterfield himself essentially became the tape recorder.”

Butterfield departed the White House in early 1973 and became head of the Federal Aviation Administration. On July 24, 1974, the Supreme Court ordered Nixon to hand over the tapes. The president resigned 15 days later.

When asked in a May 1975 People magazine interview if Nixon had contacted him since the president stepped down, Butterfield answered, “Oh no. I’m sure he hates me as much as anyone can.” In his 1978 memoir, Nixon speculates that his former aide may have been responsible for erasing the infamous 18-and-a-half minutes from the Oval Office recordings.

Woodward is the author of 17 previous books, including four on Watergate: “All the President’s Men” and “The Final Days,” both co-authored with former Post reporter Carl Bernstein, as well as “Shadow: Five Presidents and the Legacy of Watergate” (1999) and “The Secret Man: The Story of Watergate’s Deep Throat.”

“The Last of the President’s Men” is dedicated to former Washington Post executive editor Ben Bradlee, who led the Post during Watergate and passed away Oct. 21, 2014. “This is a story Ben would love, the underneath of the underneath of the whole Nixon operation,” Woodward said.

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