Robert P. Hanssen, a former F.B.I. agent who spied for Moscow on and off for greater than 20 years throughout and after the Cold War in one of the vital damaging espionage circumstances in American historical past, was found useless in his jail cell in Colorado on Monday, federal authorities introduced. He was 79.

The Federal Bureau of Prisons stated in a press release that Mr. Hanssen was discovered unresponsive simply earlier than 7 a.m. on the United States Penitentiary Florence, the place he was serving a life sentence. He was pronounced useless after lifesaving efforts by emergency medical staff. The assertion didn’t establish a trigger.

Mr. Hanssen’s case was thought of one of the vital infamous spy scandals of his era, surprising F.B.I. leaders and different authorities officers after they discovered that one in every of their very own had been feeding info to the opposite facet with impunity for thus a few years. To this present day, the F.B.I. describes him as “the most damaging spy in bureau history.”

In trade for $1.4 million in money, financial institution funds and diamonds, Mr. Hanssen handed alongside a torrent of secrets and techniques to Moscow, together with one disclosing that the United States authorities had dug a tunnel beneath the Soviet embassy in Washington to snoop on diplomatic and different communications. He additionally knowledgeable Moscow about three Okay.G.B. officers who had been secretly spying for the United States, two of whom had been later executed.

“The magnitude of Hanssen’s crimes cannot be overstated,” Paul J. McNulty, who was the U.S. legal professional who prosecuted him, stated on Monday in response to experiences of his dying. “They will long be remembered as being among the most egregious betrayals of trust in U.S. history. It was both a low point and an investigative success for the FBI.”

Mr. Hanssen’s arrest, in 2001, briefly ruptured relations between the United States and Russia at a time when the 2 former enemies had been looking for to construct friendlier ties after the collapse of the Soviet Union. President George W. Bush expelled about 50 Russian diplomats, and President Vladimir V. Putin of Russia retaliated with a tit-for-tat expulsion of fifty American diplomats. But each side had been decided to finish the matter there and never enable it to end in a extra lasting rift.

The discovery of Mr. Hanssen’s espionage embarrassed the F.B.I. and resulted in adjustments to safety procedures. He advised investigators after his arrest that safety on the bureau was so lax that it amounted to “criminal negligence.” He stated it was a easy matter to achieve entry to categorised materials on official computer systems with solely routine safety clearances.

“Any clerk in the bureau could come up with stuff on that system,” Mr. Hanssen stated, based on a Justice Department report on his case in 2002. “It’s criminal what’s laid out.”

Mr. Hanssen pleaded responsible to fifteen counts of espionage and conspiracy to keep away from the dying penalty and expressed regret for his betrayal. “I am shamed by it,” he stated throughout the 2002 listening to the place he was sentenced to life in prison with out parole.

Since July 17, 2002, Mr. Hanssen had been in custody at Florence, the supermax facility that’s thought of essentially the most safe jail within the federal system and used in recent times to deal with convicted terrorists. Inmates there are usually held in solitary confinement for 23 hours a day.

Mr. Hanssen joined the F.B.I. in 1976 as a particular agent and went on to carry a number of counterintelligence positions that gave him entry to categorised info. He started spying for the Soviet Union three years after becoming a member of the bureau, when he was assigned to a counterintelligence unit in New York, by strolling into the New York places of work of Amtorg, a Soviet commerce group that was recognized to be a entrance for the Soviet navy intelligence company.

He stopped spying for a number of years beginning in 1980, after his spouse, Bonnie, walked in on him within the basement of their residence in Westchester County, N.Y., and he rapidly tried to cowl up his papers. He confessed to her and to a priest affiliated with Opus Dei, the conservative Catholic group to which the couple belonged.

In 1985, he started spying once more, offering info to the Okay.G.B. This time he did a greater job of masking his tracks, utilizing encrypted communications and different secret strategies; even the Russians by no means knew who he was. Identifying himself solely by code names like B and Ramon Garcia, Mr. Hanssen turned over delicate info stated to incorporate particular satellite tv for pc intelligence assortment capabilities.

He stopped spying once more after the Soviet Union collapsed, then resumed once more in 1999. His betrayal went undetected for years as he collected a minimum of $600,000 in money and diamonds from the Okay.G.B. and its post-Soviet successor, S.V.R., which advised him that that they had put aside one other $800,000 for him in a Moscow financial institution, based on prosecutors.

In the Nineties, after the arrest of Aldrich Ames, a C.I.A. agent who had additionally spied for the Russians, the F.B.I. and the C.I.A. realized that another person was nonetheless offering Russia with categorised info, they usually started “Graysuit,” a hunt for the unknown double agent. But it was not till 2000 that investigators had been capable of slender the search, when the F.B.I. paid $7 million to a former Russian intelligence officer for a file on the nameless mole who known as himself B — a file that included an audio recording with a voice that two F.B.I. analysts who knew Mr. Hanssen finally acknowledged.

Using fingerprints, the F.B.I. confirmed that the mole was Mr. Hanssen and surveilled him for months, even selling him to maintain higher monitor of him. In February 2001, brokers arrested him in Foxstone Park within the Washington suburb of Vienna, Va., a couple of blocks from his residence, after he had left categorised paperwork in a rubbish bag at a “dead drop” for his Russian handlers underneath a picket footbridge.

Mr. Hanssen appeared unsurprised at lastly being caught. “What took you so long?” he reportedly requested when arrested.

Robert Philip Hanssen was born on April 18, 1944, in Chicago to Vivian and Howard Hanssen, a profession Chicago police officer who did intelligence work for the division. An solely baby who was seen as nerdy and by no means slot in, Robert had a troublesome relationship together with his father, who emotionally abused him. He grew up obsessive about James Bond, accumulating spy devices and even opening a Swiss checking account.

Mr. Hanssen obtained a bachelor’s diploma in chemistry in 1966 from Knox College in Illinois, the place he additionally studied Russian, however after commencement he was rejected by the National Security Agency when he utilized for a place in cryptography. He enrolled in dentistry faculty at Northwestern University, however later transferred to the enterprise faculty, the place he obtained a grasp’s diploma in enterprise administration.

While in dentistry faculty, he met and married Bonnie Wauck and transformed from Lutheran to hitch her Roman Catholic religion. After a 12 months working at an accounting agency, he took a place with the Chicago Police Department specializing in forensic accounting. Four years later he moved to the F.B.I.

Bright however brittle, Mr. Hanssen was stated to have burned with resentment that he didn’t obtain the respect and assignments he felt he deserved. With six kids in parochial colleges or school, he attributed his resolution to spy for Moscow to cash, though his causes had been by no means absolutely understood.

“Many of the factors that have motivated or influenced traitors in the past — such as greed, ideology, career disappointments and resentment, and drug and alcohol abuse — do not apply to Hanssen or do not fully explain his conduct,” a Justice Department inspector general’s report on the case said in 2003.

Mr. Hanssen led a double life in additional methods than one. An lively member of the Roman Catholic lay group Opus Dei, he introduced himself as a non secular and dedicated anti-communist conservative. But based on experiences, he additionally visited strip golf equipment, allowed a buddy to clandestinely watch him having intercourse together with his spouse and engaged in what was stated to be a secret however nonsexual relationship with an unique dancer whom he plied with presents and took on an F.B.I. journey to Hong Kong.

Mr. Hanssen’s capacity to keep away from detection was a sign failure of the American intelligence equipment. His personal brother-in-law, who additionally labored for the F.B.I., reported suspicions about Mr. Hanssen to the bureau a decade earlier than his arrest, however the supervisor he advised had dismissed his considerations.

Mr. Hanssen was the topic of a number of books and movies, together with a tv film in 2002 during which he was performed by William Hurt and a full-screen film known as “Breach” in 2007, during which he was performed by Chris Cooper.

“Hanssen was a thicket of paradoxes, a suburban dad and outwardly devoted family man who professed to be deeply religious while at the same time betraying family, faith and country, all and everyone who ever mattered to him,” Ann Blackman, a co-author of “The Spy Next Door,” stated on Monday. “For 21 years, through the terms of four presidents and three F.B.I. directors, he fooled them all.”

Jesus Jiménez contributed reporting from New York.