On an afternoon in August 2001, a team of FBI agents halted a tram on the tarmac of Washington Dulles International Airport and removed a passenger named Brian Regan, just moments before he boarded a flight bound for Switzerland. Regan was an employee of the top-secret National Reconnaissance Office (NRO), the government agency responsible for America’s intelligence satellites, and he had on him pages filled with trinomials — groups of three-character sequences that anyone in the clandestine services would quickly recognize as potentially coded information.

If Regan were able to pass those trinomials to an enemy of America, as he had planned, they would lead the decoder to an enormous wealth of classified information. The collaring of Regan was the result of a long investigation spearheaded by FBI agent Steven Carr, comprehensively and thrillingly detailed by Yudhijit Bhattacharjee in “The Spy Who Couldn’t Spell: A Dyslexic Traitor, an Unbreakable Code, and the FBI’s Hunt for America’s Stolen Secrets.”

Brian Patrick Regan grew up in a large Irish Catholic family in Farmingdale. A doltish, socially awkward kid, he received additional ridicule for his learning disability, later diagnosed as dyslexia. After foundering in school, he ended up in the Air Force, where he was able to develop a skill for code-breaking and soon move into the NRO as a signals intelligence specialist. After a lackluster career during which anti-social behavior hindered his advancement — and some stunningly poor financial decisions left him and his family in enormous debt — he was prepared to betray his country to the highest bidder.

Working methodically over many months, Regan printed out thousands of pages of extremely sensitive images and information from an intelligence database to which he had access. He divided the pages into many separate caches, which he then placed in plastic bags and buried in public parks across Maryland and Virginia. Next, he devised a system to encode the locations of these “dead drops,” then encoded that information in yet another code, and buried these codes in small plastic capsules of their own.

The key to the puzzle, a laminated phone list that would act as a code book, Regan buried on the grounds of Farmingdale High School, where he had been bullied years before. Once he had made contact with an interested client state (he reached out to Libya, Iraq and China), they would be able to find the drops on their own, leaving his anonymity intact. The damage to American national interests would have been catastrophic.

In December 2000, the FBI’s Steven Carr received a package at his office in Washington, D.C., from a confidential informant in the Libyan consulate in New York. The package contained three different letters that the informant had intercepted from an anonymous source, the only readable components of which were the cover sheets requesting that the contents be passed via diplomatic pouch to a Libyan intelligence chief. One letter contained four pages of alphanumeric code, and the other two contained ciphers and instructions for decoding the first.

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Bhattacharjee tells us, “Carr’s pulse quickened as he read the deciphered text. ‘I am a Middle East North African analyst for the Central Intelligence Agency. I am willing to commit espionage against the U.S. by providing your country with highly classified information.” Carr, the author writes, “might as well have been looking at a warning sign for a national security threat flashing in neon red.” In addition to the treasonous overture, the letter contained a feature that befuddled Carr and his colleagues: repeated and consistent misspellings (“precausion,” “anonmus,” “airbourn”).

Carr’s race to uncover Regan before he can execute his scheme is the stuff of brainy Hollywood thrillers. The codes and methods that Regan used to mask his activity were, in Bhattacharjee’s words, “cunning and diabolical,” but in the end he was undone by his propensity to make minor mistakes that Carr could then exploit, peeling the layers of secrecy back one by one until that climactic race against time on the Dulles tarmac.

Bhattacharjee, a former staff writer for the journal Science, scrupulously re-creates Carr’s work in deciphering each code so that the reader can follow along. The result is a full appreciation for the guile, initiative and determination Carr employed to obtain a conviction and life sentence for a criminal who put countless American lives at risk. This fascinating incident from our nation’s history, and the lessons it provides, ought to be well-remembered, and “The Spy Who Couldn’t Spell” deserves to be widely read.