Alexander Butterfield, the White House aide who inadvertently hastened Richard Nixon’s resignation over the Watergate scandal when he revealed that the president had bugged the Oval Office and Cabinet Room and routinely recorded his conversations, has died. He was 99.

His death was confirmed to The Associated Press by his wife, Kim, and John Dean, who served as White House counsel to Nixon during the Watergate scandal and went on to, along with Butterfield, help expose the wrongdoing.

“He had the heavy responsibility of revealing something he was sworn to secrecy on, which is the installation of the Nixon taping system,” Dean said. “He stood up and told the truth.”

As a deputy assistant to the president, Butterfield oversaw the taping system connected to voice-activated listening devices that had been secretly placed in four locations, including Nixon’s office in the Executive Office Building and the presidential retreat at Camp David.

Butterfield later said that, besides himself and the president, he believed that only White House chief of staff H.R. Haldeman, a Haldeman assistant and a handful of Secret Service agents knew about the taping system.

“Everything was taped … as long as the president was in attendance,” Butterfield told Watergate investigators when testifying under oath during a preliminary interview.

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The tapes would expose Nixon’s role in the cover-up that followed the burglary in 1972 at the Democratic Party headquarters at the Watergate building. To avoid impeachment by the House, Nixon resigned on Aug. 9, 1974, less than a month after the Supreme Court had ordered him to surrender the relevant tapes to the Watergate special prosecutor.

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Butterfield believed he’d had a hand in the president’s fate. “I didn’t like to be the cause of that, but I felt that I was, in a lot of ways,” he said in a 2008 oral history for the Nixon Presidential Library and Museum.

Butterfield, a college friend of Haldeman’s at UCLA who had contacted his friend to ask about opportunities in the new Nixon administration, served as a deputy assistant to Nixon from 1969 to 1973. In that capacity, he worked under Haldeman and, among other duties, was secretary to the Cabinet and helped oversee White House operations.

The Air Force veteran had left the White House to become administrator of the Federal Aviation Administration when Senate committee staffers privately questioned him on July 13, 1973, during their investigation of the Watergate break-in.

A routine question about the possibility of a taping system had been prompted by former White House counsel John Dean’s testimony that he believed a conversation he had had with Nixon may have been recorded.

When Butterfield acknowledged that a taping system indeed existed, he was brought before a public hearing of the Senate Select Committee on Presidential Campaign Activities.

The public revelation on July 16, 1973, of a taping system designed to record all the president’s conversations stunned Nixon friends and foes alike. The tapes promised Watergate investigators a rich vein of evidence in their quest to determine what Nixon and others knew about the break-in — a great deal, as it turned out.

Efforts by investigators to gain access to the tapes sparked a yearlong legal battle that was resolved in July 1974 when the Supreme Court ruled unanimously that Nixon had to give them up.

The thousands of hours of tapes made public over the years — they are now controlled by the National Archives — provide a unique, if often unflattering, view of Nixon.

His words exposed a bad temper, vulgar language, bigoted racial and religious views, and unvarnished opinions about national and international figures.

“I just thought, ‘When they hear those tapes …’ I mean, I knew what was on these tapes … they’re dynamite,” Butterfield told the Nixon Library. “I guess I didn’t foresee that the president might be put out of office or impeached, but I thought it would be a perilous few years for him. I guess I couldn’t conceive of (Nixon) being forced out of office. It had never happened before.”

Butterfield later said he believed that Nixon’s successor, President Gerald Ford, fired him as FAA administrator in 1975 as part of an agreement worked out between the Nixon and Ford staff members.

He said he had heard from White House friends that he had been targeted shortly after his testimony to the Senate committee.

After leaving the FAA, Butterfield worked as a business executive in California. He earned a master’s degree from the University of California, San Diego, in 1994.

Alexander Porter Butterfield was born on April 6, 1926, in Pensacola, Florida.

He left UCLA to join the Navy and later earned a bachelor’s degree from the University of Maryland in 1956 and a master’s degree from George Washington University in 1967.

In 1948, he joined the Air Force and served as an instructor at a base near Las Vegas during the Korean War and later served in Germany.

In Washington, he was a military assistant to the special assistant of the defence secretary in 1965 and 1966 and later served as senior military representative of the U.S. and representative for the commander-in-chief, Pacific Forces, Australia.

He retired at the rank of colonel after 20 years in the Air Force.

Butterfield was unsparing in his criticism of the former president in later years. While he commended Nixon’s achievements in foreign affairs, he considered his former boss “not an honest man” and “a crook” and believed that Nixon knew about the Watergate break-in before it occurred and was the architect of the ensuing cover-up.

Butterfield found himself “cheering … just cheering” the day Nixon resigned, he told the Nixon Library, because “justice had prevailed.”

“I didn’t think that it would for a while,” he said. “This guy was the ringleader.”