Richard Pipes revealed a mild-mannered US maths professor as a former Russian revolutionary who betrayed his comrades then killed his boss

In the autumn of 1897, a recent Johns Hopkins University PhD by the name of Alexander Pell arrived at the University of South Dakota to assume duties as professor of mathematics. Of his background little was known. Pell told his colleagues that his original name had been Alexander Polevoi; he had adopted his new name on taking US citizenship in 1891.

Although he could not rid himself of his Russian "brogue", Pell made a conscious effort to assimilate. At home, he spoke only English and is said to have voted Republican. Assuming a heavy teaching load, he also took part in a variety of extracurricular activities, including chess and sports. He became highly popular. One alumnus recalled: "Dr Pell occupied a unique position in the minds and hearts of his students. He was one of the most human men I have ever known." No one suspected that "Jolly Little Pell" was in reality Sergei Degaev, a one-time murderer and police informer.

As a professional historian, I have tended to focus on "big" issues: the dissolution of the Russian empire in 1917-18 and the emergence of the Soviet state; the evolution of the political institutions of Imperial Russia; the Russian Revolution. But once in a while I have been attracted to individuals who, for one reason or another, appeared to personify their age. Degaev seemed to me to reflect the baffling contradictions of his historical era.

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He was born in 1857 in the family of an army physician and was destined for a military career, but in his early 20s he became involved in revolutionary activity. He joined the People's Will, the first political organisation committed to terror as a means of achieving radical change in Russia's political and social order. When the security services learnt of his involvement they forced him to resign from active military service. Degaev then became a full-time revolutionary.

The terrorist cause in Russia acquired great popularity in about 1880 - not among the peasants and workers who made up some 85 per cent of the population but among educated urbanites, who seem to have believed that assassinations would force the country's leaders to give up autocratic powers.

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Degaev faithfully served the People's Will, yet never made it to the top of its highly centralised organisational hierarchy because he was considered emotionally unsuited for terrorist activity owing to his "squeamishness".

In March 1881, the People's Will realised its immediate objective - the assassination of Tsar Alexander II. In the aftermath, the police decimated the organisation's ranks, arresting, executing and exiling most of its members. The few principals who managed to elude capture fled abroad.

Degaev, who had the good fortune to remain at liberty, gradually rose to a position of leadership in what was left of the organisation.

But in December 1882 he too was arrested as he was setting up an illegal printing press in Odessa. In prison, Degaev concluded that the cause of political terrorism was doomed and that radical change would be better served by collaboration with reform-minded members of the tsarist establishment.

One such figure was Lieutenant Colonel Georgii Sudeikin, who at the time of Degaev's arrest, aged 32, had been placed in charge of internal security for the empire. He was, in fact, the man responsible for breaking up the People's Will and ensuring the safety of the new tsar, Alexander III.

Sudeikin accomplished this by penetrating the terrorist organisation with spies. Many of his operatives were revolutionaries whom he had convinced that he shared common ideals that would come about only if they abandoned violence.

In reality, he was an unprincipled careerist without any loyalty, even to the authorities he served. Sudeikin was frustrated, however, by the refusal of the authorities to promote him to general. He needed this rank to qualify for a personal audience with the tsar, which he was convinced would result in his becoming a close imperial advisor.

Degaev had had casual contact with Sudeikin before his arrest. He now wrote him a letter from prison suggesting they meet. Sudeikin hurried to Odessa and learnt that Degaev was prepared to collaborate. Degaev promptly revealed the names of all the terrorists and sympathisers of whom he had knowledge. Sudeikin arranged for Degaev's escape, and then entered into a partnership with him that must have been unique in revolutionary annals.

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Sudeikin proposed that Degaev, with the knowledge and assistance of the security services, take over the remnant of the People's Will and steer it away from further attempts on the life of the tsar. He would also organise the assassination of several high government officials, including Dmitry Tolstoy, the minister of the interior, whom Sudeikin blamed for his failure to advance in rank.

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This police-controlled terrorism, the ambitious official believed, would so frighten the court that it would appoint him virtual dictator. Degaev was to arrange for a fake attempt on Sudeikin's life that would inflict wounds that would enable the latter to be out of the picture when the People's Will assassinated his designated victims.

In May 1883, whether through pangs of conscience, fear of exposure or suspicion that Sudeikin was using him for his own purposes, Degaev had a change of heart. He travelled to Paris and confessed his treachery to the leaders of the People's Will. He was told that he deserved a traitor's death but that his life would be spared if he personally killed Sudeikin.

Degaev agreed, but on his return to Russia continued to collaborate with the police for another six months before finally proceeding with the plot.

With the help of two thugs selected by the People's Will, Degaev lured Sudeikin to his apartment: there he shot him in the back while his accomplices finished him off with iron bars.

The revolutionaries smuggled Degaev to Paris, where he was "tried" by their leaders. They ordered him to abandon any further connection with the revolutionary cause and emigrate overseas. So in 1886 he arrived with his wife in the US, and held down a variety of jobs, including management of a chemical factory in St Louis, Missouri. In 1891, he became a US citizen under the name Alexander Pell, an alias he may have adopted from the 17th-century English mathematician John Pell.

For Degaev's true passion was mathematics. While earning a living in St Louis he audited mathematics courses at the local university and in 1895 enrolled at Johns Hopkins for a doctorate. It was granted two years later, whereupon the newly founded University of South Dakota offered him a professorship.

There he became a much-loved academic. But he lived in constant fear of exposure and had his younger brother, then working for the Russian consulate in New York, place notices in newspapers announcing his death.

In 1921, Degaev died peacefully. His last recorded words, uttered in 1918 after the proclamation of Lenin's "Red Terror", were: "Accursed Russia, even after liberating herself, she does not let people live."

In a thriller, the author knows the plot but conceals it from the reader so as to keep him in suspense. In the case of Degaev, my knowledge of the plot is limited to a sequence of events while the motives of the protagonists remain concealed. Neither Degaev nor Sudeikin left behind little more than scraps of written evidence: the latter was so successful in concealing his identity that it was only three years ago that a Russian scholar located and published his photograph. Degaev tried to write his memoirs but quickly gave up.

Thus there remain numerous questions to be resolved about this extraordinary episode in revolutionary history. Why did Degaev, given his aversion to spilling blood, join the People's Will? How did the head of the security police and a prominent revolutionary gain each other's confidence? Why did Degaev, having disclosed his treason, wait another six months to assassinate Sudeikin? How did he manage to adapt so readily to life on what was then the American frontier? And, last but not least, how would his life have evolved had he been born in America rather than Russia: would he have become a gentle academic instead of a terrorist and renegade?

In the short book I have written about Degaev, I am keenly aware at all stages how little I know and understand and ask my readers to participate in solving the many riddles presented by the lives of its protagonists.

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Richard Pipes is emeritus professor of history at Harvard University. His book The Degaev Affair: Terror and Treason in Tsarist Russia is published by Yale University Press (?15.95).