Lilia Estrin became Sedov's secretary. She wrote to his mother, Natalia Sedova: "I work like an ox, from early morning to late into the night, and I am content. My job (with the Institute) is interesting. After it, I work for the Bulletin and other (Trotskyite) matters which keep me up until one o'clock at night. At seven in the morning I am up again... I need no Sundays, no respite. I am a dynamic person, I need action."

According to Robert Service, the author of Trotsky (2009), some of the group were highly suspicious of Mark Zborowski (Etienne): "His story was that he was a committed Trotskyist from Ukraine who had travelled to France in 1933 to offer his services. He retained Lev's complete confidence despite the reservations expressed by French comrades. Etienne aimed to become indispensable to Lev, and he succeeded. Cool-headed and assiduous, he relieved Lev of many tasks in a heavy workload. Not everyone took to him. It was far from clear where he got his money from or even how he managed to subsist. Lev's secretary Lilia Estrin sympathetically invented jobs for him to do and paid him each time he did one of them. A routine was established: Etienne worked alongside Lev in the mornings and Lilia took his place in the afternoons. Equipped by his handlers with a camera, Etienne photographed items in the organization's files.... Etienne's growing prominence in this situation gave rise to suspicions among French Trotskyists." Pierre Naville mentioned his worries to Trotsky, who retorted: "You want to deprive me of my collaborators."

Walter Krivitsky was an NKVD agent who decided to defect after the recall and execution of agents such as Theodore Maly and Vladimir Antonov-Ovseenko. He arranged to meet Lev Sedov in the company of Fedor Dan and warned him that there was an informer within his group. Krivitsky suggested that it might be Mark Zborowski. According to Gary Kern, the author of A Death in Washington: Walter G. Krivitsky and the Stalin Terror (2004): "Krivitsky had no desire to join the Trotskyists, but was impressed by Sedov, admiring his revolutionary fervor, hard work and austere life style."

Victor Serge has pointed out that towards the end of 1937 Sedov suffered from ill health. "For several months Sedov had been complaining of various indispositions, in particular of a rather high temperature in the evenings. He wasn’t able to stand up to such ill-health. He had been leading a hard life, every hour taken up by resistance to the most extensive and sinister intrigues of contemporary history – those of a regime of foul terror born out of the dictatorship of the proletariat. It was obvious that his physical strength was exhausted. His spirits were good, the indestructible spirits of a young revolutionary for whom socialist activity is not an optional extra but his very reason for living, and who has committed himself in an age of defeat and demoralisation, without illusions and like a man."

Lev Sedov had severe stomach pains. On 9th February, he was taken by Mark Zborowski to the Bergere Clinic, a small establishment run by Russian émigrés connected with the Union for Repatriation of Russians Abroad in Paris. Sedov had a operation for appendicitis that evening. It was claimed that the operation was successful and was making a good recovery. However, according to Bertrand M. Patenaude, the author of Stalin's Nemesis: The Exile and Murder of Leon Trotsky (2009): "The patient appeared to be recuperating well, until the night of 13-14 February, when he was seen wandering the unattended corridors, half-naked and raving in Russian. He was discovered in the morning lying on a bed in a nearby office, critically ill. His bed and his room were soiled with excrement. A second operation was performed on the evening of 15 February, but after enduring hours of agonizing pain, the patient died the following morning."

Edward P. Gazur, the author of Alexander Orlov: The FBI's KGB General (2001) has argued that Alexander Orlov believed he was murdered: "What concerned Orlov greatly was the fact that the hospital Sedov had been taken to, and where he expired, was the small clinic of Professor Bergere in Paris. Exactly a year earlier, Orlov had been in the same clinic because of his car accident while at the front. He had been cared for at the Bergere Clinic because it was a hospital that was trusted by the KGB to take care of high-ranking Soviet officials. Professor Bergere and his staff were sympathetic towards the Communist cause and under the influence of the KGB. Orlov was in Spain at the time of Sedov's death and was unable to ascertain the complete facts, but speculated that at the moment the KGB Centre had been apprised of the circumstances by Mark, the decision had been made to take advantage of the situation and eliminate Sedov. The autopsy performed by the KGB hirelings had to have been bogus to conceal the true cause of death."

Leon Trotsky was devastated by the death of his eldest son. In a press release on 18th February he stated: "He was not only my son but my best friend." Trotsky received information from several sources that Mark Zborowski was an NKVD agent. He asked Rudolf Klement to carry out an investigation of Zborowski. According to Gary Kern "Klement put together a file and planned to take it to Brussels on July 14, where he would circulate it among various branches of the Opposition. But no one in Brussels ever saw him." Klement's headless corpse was washed ashore in August 1938. He was identified by a friend from peculiar scars and marks on the body.

Lilia Estrin was with Trotsky in Mexico in 1939 when he received an anonymous letter warning him that a spy named Mark had infiltrated the Parisian group. The only person of that name was Zborowski. According to Gary Kern, the author of A Death in Washington: Walter G. Krivitsky and the Stalin Terror (2004): "Both of them discounted it because other letters (probably sent by the NKVD) had warned Trotsky against other intimates of his circle, including Lilia.... He realized that he could not spend all of his time investigating every member of his staff, yet failure to do so would leave him defenseless against infiltration. Even though he believed that his son had been murdered by the NKVD, he refused to act on the letter; Lilia returned to Paris and told Zborowski all about it." Lilia discovered several years later that the letter had come from Alexander Orlov.

When the German Army invaded France in May 1940 Lelia fled to the United States. Soon afterwards she married David Dallin. A few months later Mark Zborowski arrived in New York City. He immediately contacted the Dallins. They helped him find employment at a factory in Brooklyn and set him up in an apartment. A few months later he moved to a more expensive home at 201 West 108th Street, where the Dallins also lived. It was later discovered that the NKVD were paying Zborowski to spy on the Dallins.

In 1954, Dallin had a meeting with Alexander Orlov. He wanted advice on a book he was writing. During the conversation Orlov asked Dallin if he knew "Mark, the agent provocateur" who was a member of the Left Opposition on Paris in the 1930s? Orlov said that as an NKVD agent he had read Mark's reports on the group. Dallin said that the only man he knew of that name was Mark Zborowski.

The next meeting between the two men took place on 25th December, 1954. This time Lilia Dallin attended. Orlov told Lilia that when Lev Sedov was at the Bergere Clinic "Mark" sent a report to the NKVD that he had a tremendous urge for an orange and that it was provided by Lilia. This was a true and Lilia now came to the conclusion that Mark Zborowski was indeed a Soviet agent and told Orlov that his suspicions must be correct. Two days later Orlov told the FBI that there was a known Soviet agent in the United States.

The former NKVD agent, Alexander Orlov, appeared before the Senate Internal Security Subcommittee in September 1955. He disclosed that Mark Zborowski had been involved in the killing of Ignaz Reiss and Lev Sedov. Zborowski appeared before the committee in February 1956. He admitted to being a Soviet agent working against the supporters of Leon Trotsky in Europe in the 1930s but denied that he had continued these activities in the United States. Lilia Dallin appeared before the committee in March 1956. She also gave information against Zborowski. However, it was not until November 1962, that he was convicted of perjury and received a four-year prison sentence.

Lilia Estrin Dallin died in 1981.