After 237 years, it now can be said with certainty: British spymaster John André spent time in Oyster Bay during the American Revolution.

Claire Bellerjeau, resident historian at the hamlet’s historic Raynham Hall Museum, tracked down letters and an intelligence report written by André that proved he was there for a prolonged visit in 1779 and met with another espionage agent there. She will present her findings Saturday afternoon at the Matinecock Lodge next to the museum, followed by the grand opening of the “Spies’ Nest” exhibit at Raynham Hall.

Her sleuthing confirmed American Revolution historians’ long-held belief that the espionage chief was in Oyster Bay when the Samuel Townsend home served as British headquarters — while Townsend’s son, Robert, coincidentally was one of the top agents in Gen. George Washington’s Culper Spy Ring.

Bellerjeau “appears to have unearthed an astonishing discovery that, after nearly a quarter of a millennium, turns rumor into fact,” said Alexander Rose, author of “Washington’s Spies: The Story of America’s First Spy Ring.” “Raynham Hall played a far greater role in the American Revolution than traditionally thought.”

Since 1839, historians have suggested that André visited the Townsend home shortly before he was hanged in 1780 for his part in Benedict Arnold’s treason plot. Some authors had included vague references to letters that placed André in Oyster Bay, Bellerjeau said, raising questions in her mind.

Harriet Gerard Clark, left, executive director of Raynham Hall Museum in Oyster Bay, and Claire Bellerjeau, a historian and researcher at the museum, on Sunday, Oct. 9, 2016. Clark holds two portraits: The painting on the left is of British Col. John Graves Simcoe, who made Raynham Hall -- the home of Samuel Townsend -- his residence and headquarters from November 1778 to May 1779. The other painting is of Capt. John André, a British spymaster. Photo Credit: Steven Sunshine

Over the past year, she located a 1779 letter and an intelligence report that André sent from Oyster Bay to the British commander, Gen. Henry Clinton. She found two more letters that André wrote to Col. John Graves Simcoe, commander of the Queen’s Rangers, who made the Townsend house his headquarters from November 1778 to May 1779.

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The two letters to Simcoe also made it clear that André spent time at Raynham Hall. Written elsewhere before and after André’s time in Oyster Bay, both were personal in nature and filled with rich details, providing an intriguing window into the close relationship between the two.

“This shows,” Raynham Hall executive director Harriet Gerard Clark said, “that Oyster Bay has very deep and interesting roots that extend for hundreds of years.”

Bellerjeau began her detective work last November, researching Simcoe’s papers at the Ontario Archives in Toronto. She came across a microfilm copy of a Feb. 29, 1779, letter — never mentioned in any book — by André to Simcoe in Oyster Bay. He discusses seeing Simcoe at a country house occupied by Clinton, possibly in East Hampton, and his hope of visiting Simcoe in Oyster Bay.

During the summer, Bellerjeau traveled to the University of Michigan, where she found a March 20, 1779, letter that first was briefly referenced in Carl Van Doren’s 1941 book “Secret History of the American Revolution.”

That letter, which states it was written in Oyster Bay, discusses André’s rendezvous with British spy Christopher Sower Jr. In the correspondence to Clinton in East Hampton, André — a captain soon to be promoted to major — states that Sower’s letter contained so much information that “I thought proper to detain it” and instead send along a summary with his own cover letter, because the original was “detailed in such a manner that should it fall into improper hands a train of correspondence with names and etc. would have been laid open.”

André planned to see Clinton and verbally give him all the details he was editing out of the Sower letter.

In that March 20 letter, André talks about being in Oyster Bay. “I have found myself situated here with some conveniency for attending to my health and shall remain within place until the weather mends,” he wrote. He also mentioned “a certain event taking place” and said, “I shall also keep on the watch . . . ”

Bellerjeau believes that event might have been the upcoming wedding of Arnold to loyalist Peggy Shippen, who facilitated the American general’s defection to the British the following year.

She found information about the third letter — from André to Simcoe — in a 1922 catalog from a London bookseller.

The April 6, 1779, letter, mentioned briefly in some histories, was partially transcribed in the catalog Bellerjeau purchased online. It talks about André still being ill and recuperating at one of Clinton’s country houses and having left Oyster Bay and gone to New York.

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Bellerjeau said the whereabouts of the actual letter is unknown.

“I think for the history of Raynham Hall, it shows not just that these important historical figures were here but it gives us a window into their world,” she said.