At about 9:00 Moscow time on August 13, a 33-year-old Russian woman named Maria Pevchikh checked in for a flight from Moscow to the Siberian city of Novosibirsk. She was not alone.

Trailing her to Domodedovo airport that morning was a member of an elite unit of the Russian Security Service, the FSB. Oleg Tayakin, a slim, balding man with blue-green eyes, remained at the airport until Pevchikh left. The woman was traveling in advance of a visit to Siberia by Russian opposition leader Alexey Navalny, whose anti-corruption campaign she leads, CNN reported.

The Navalny team is constantly watched by the FSB in Russia. But Tayakin is no ordinary agent. He belongs to a small team specializing in toxins and nerve agents. That very morning, several of its agents were on their way to Novosibirsk, two hours ahead of Pevchikh. They knew that Navalny, a thorn in President Vladimir Putin's side for nearly a decade, would arrive in Novosibirsk the next day.

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Exactly one week later, Navalny would be fighting for his life -- his body ravaged by the nerve agent Novichok, his organs shutting down. He collapsed on a flight from Tomsk, the last stop on his Siberian trip.

Read alsoEU imposes sanctions on Russian military intelligence chief – mediaCNN joined an investigation by the group Bellingcat that has pieced together how the elite FSB unit followed Navalny's team throughout its August trip to Siberia.

The investigation also found that this unit has followed Navalny on more than 30 trips to and from Moscow since 2017.

By examining thousands of phone records along with flight manifests and other documents obtained by Bellingcat, this joint months-long investigation has identified the agents involved, as well as their backgrounds, communications and travel. CNN showed photographs of several of the agents to Navalny last week, during an interview at a secret location in Germany, where he is still recovering.

He said he did not recognize any of them, and then paused.

"I have a very strange feeling when I watch their faces," Navalny said, adding it was "absolutely terrifying" to find out he'd been followed for so long.

Bellingcat and CNN have established that the FSB's toxins team comprises six to 10 agents, including qualified doctors, toxicologists and paramedics. The FSB agents trailing Navalny -- in their late 30s and 40s – usually traveled in groups of three, taking parallel flights as they tracked Navalny. More recently they began using burner phones.

Their special unit is based at a nondescript beige-colored compound on Akademika Vargi Street in the southwest outskirts of Moscow: the headquarters of the FSB's Criminalistics Institute. The group sometimes works out of another secure compound on the eastern edge of Moscow. The official in direct command of the unit is Stanislav Makshakov. He held the rank of colonel when he worked at the Shikhany Institute near Saratov in southwest Russia, according to others who worked there. It was there that the nerve agent Novichok was developed in the 1970s and produced over the next two decades. Makshakov also holds a patent relating to mustard gas.

While Navalny may never have seen his shadows, this investigation found that other parts of their tradecraft were not that elaborate. Until 2018, agents in the unit frequently used their own names as they followed their target. Some then adopted fake identities but used their wives' maiden names and changed their dates of birth by a year.

Cell phone data shows that, in the weeks before Navalny's poisoning, Makshakov and Maj. Gen. Vladimir Bogdanov, commander of the Center for Special Equipment at the FSB, communicated regularly with specialists researching nerve agents.

Bogdanov is a very senior figure at the FSB. Cell phone data shows he was also in touch with a senior Kremlin official and confidante of Putin on July 2. The very next day, Navalny and his wife, Yulia, began a brief vacation at a hotel near Kaliningrad, a Russian province sandwiched between Lithuania and Poland on the Baltic Sea. Flight manifests obtained by the investigation show at least three members of the FSB unit flew to Kaliningrad at the same time. Security cameras at the hotel were turned off for the duration of their stay, a source has told Bellingcat.

Read alsoToothless sanctions over Navalny poisoning. Why Russia will continue to use NovichokOn July 6, hours after the FSB team returned to Moscow, Yulia fell ill. Navalny told CNN that she described a sense of sudden exhaustion and disorientation. Yulia recovered and the exact cause of her illness was not determined. Experts have told CNN that such symptoms are consistent with a low dosage of poisoning. And looking back, Navalny believes the symptoms were "absolutely the same" as those he would suffer weeks later.

"I couldn't connect these dots. Now I realize how bad she was, what it was, the kind of terrible, terrible feeling she experienced at this time."

After the Kaliningrad trip, the cell phone data of at least two dozen Russian officials show a surge in communications. Among those in contact were leaders of the FSB unit at the Vargi Street compound, scientific institutes and senior FSB officials.

At the center of the web was the Signal Institute in Moscow, previously identified as at the heart of Russia's illicit and secret program to develop Novichok. Its head, Artur Zhirov, was in regular contact with officers running the FSB team, including Makshakov. Zhirov was also in touch with a St. Petersburg-based researcher, Sergey Chepur, who had close contacts with the Russian GRU military intel squad that tried to poison spy turncoat Sergey Skripal and his daughter in UK.

The probe has revealed that two teams of five to six agents were deployed on Navalny's Siberian trip. Cell phone data shows that Pevchikh, Navalny's senior aide, was under surveillance at her hotel in Novosibirsk, where the Navalny team intended to film an anti-corruption report.

After tracking Pevchikh and then Navalny to the Moscow airport, agent Tayakin remained at the headquarters on Vargi Street throughout Navalny's six-day trip, communicating constantly with the teams on the ground and apparently napping at the office whenever he could.

From Novosibirsk, the Navalny team drove to the city of Tomsk to meet opposition activists. On the night of August 19, there is one ping from a phone belonging to Alexei Alexandrov, a 39-year-old FSB operative, who was one of the toxins team. His phone's ping came from just a few blocks north of the Xander Hotel, where the Navalny team was staying.

Navalny doesn't know how he was poisoned that night. He told CNN that in the hotel's Velvet bar -- at around 11 p.m. -- he'd taken one taste of a cocktail, "which was really, really bad. And I sip a couple of times and just put it on the table and go back to my room." Equally, it could have been added to laundry he had done at the hotel, placed on a towel or pillow case or injected into a shampoo bottle.

Experts in nerve agents say the assailants deployed a previously unused variant of Novichok, likely A242 or A262. They can be spread in a solid form and are highly toxic. Depending on the dosage and how it's administered, Novichok can take up to 12 hours to impact the nervous system, according to experts in toxicology.

When Navalny felt sick on board the flight to Moscow, the crew didn’t hesitate to make an emergency landing in Omsk where Navalny was rushed to a local hospital, while news of what had happened made it to his team still in Tomsk, including Pevchikh. They scrambled to recover any evidence they could from Navalny's hotel room: a towel, water and shampoo bottles, a toothbrush.

At the same time, alarm bells were beginning to sound in Moscow. Cell phone data shows a rapid sequence of calls between the FSB leadership and two operatives of the FSB unit that had been tracking Navalny -- the unit commander Makshakov and Tayakin. Just an hour after Navalny's plane landed in Omsk, FSB commander Bogdanov called Makshakov. If the intent was to kill Navalny -- and all the toxicologists consulted by CNN believe that was the case -- the calls appear to be evidence of an effort to assess the situation and plan the next moves.