Tomsk, Russia(CNN) At about 9 a.m. 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.
Pevchikh was traveling in advance of a visit to Siberia by Russian opposition leader Alexey Navalny, whose anti-corruption campaign she leads.
The Navalny team is constantly watched by the FSB in Russia, the successor to the Soviet Union's secret police force, the KGB. 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.
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.
CNN 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.
In recent years the online investigative outfit Bellingcat has identified the Russian military intelligence agents sent to England to poison former double agent Sergei Skripal and his daughter, and a Russian agent accused of murdering a Chechen activist in Berlin. Bellingcat helped uncover the pro-Russian separatists in Ukraine responsible for shooting down the MH-17 airliner in 2014.
Russia has denied involvement in every case.
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. The investigation also involved German magazine Der Spiegel and Russian online publication The Insider.
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 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, according to public records.
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.
That lax approach to spy craft, as well as official corruption and poor data protection, have caused major embarrassment to Russian intelligence services before. Leaked databases from government registries are widely available in Russia for everything from vehicle registrations, passport details and other personal data.
Bellingcat researchers and Russian investigative journalists have used such data to reveal details about the activities of Russian hackers and intelligence operatives, who have often made basic errors such as registering their cars to their official workplaces.
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.
On 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 Akademika 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 contact with a St. Petersburg scientist, Sergey Chepur, who had close contacts with the Russian military intelligence (GRU) team that carried out the Novichok attack on the Skripals, who both survived.
Amid this flurry of activity were at least two trips by members of the FSB unit to the Black Sea resort of Sochi, where the Russian leadership spends much of the summer. Both trips were booked on short notice, according to flight manifests. The second, which lasted just a few hours, was the day before Navalny was poisoned in the city of Tomsk.
While Bellingcat and CNN cannot identify whom the travelers met in Sochi, the trip suggests that the Tomsk operation may have been approved at the highest levels.
The investigation has found that two teams comprising five or six agents were deployed on Navalny's Siberian trip. Cell phone data shows that Pevchikh, the senior aide to Navalny, was under surveillance at her hotel in Novosibirsk, where the Navalny team planned to film an anti-corruption investigation.
Navalny has focused on exposing corruption among the Russian hierarchy and oligarchs since being denied the opportunity to run against Putin in the 2018 presidential election, and encouraged tactical voting in regional elections against the governing United Russia party.
After tracking Pevchikh and then Navalny to the airport in Moscow, agent Tayakin remained at the Akademika Vargi Street headquarters day and night for the duration of Navalny's six-day Siberia trip, communicating constantly with the teams in the field and apparently napping at the office when 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. The 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."
That might have been the source.
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. Short of injecting exactly the right dose into someone, it is almost impossible for the perpetrator to dose Novichok so as to incapacitate rather than kill.
Early the next morning, Navalny left for the four-hour flight back to Moscow. He drank tea in the airport café, and began watching his favorite TV show, "Rick and Morty," soon after takeoff.
Then he felt suddenly very sick. Sweating profusely, he went to the plane's bathroom.
"I went over to the flight attendant and said, 'I was poisoned, I'm going to die.' And then I laid down under his feet to die, you knew in that moment the worst," he said.
He's still alive thanks to the quick thinking of the plane's captain, who diverted to Omsk and requested medical help, rather than continue to Moscow.
"They saved my life," Navalny said. "Because of them, I had this shot with atropine or other medicine. And I have access to the breathing machine."
As Navalny laid comatose in an Omsk hospital, 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 officers 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 a scramble to assess the situation and plan the next steps.
There seems to have been debate about whether Navalny should be allowed to leave the country for treatment in Germany.
Initially, Navalny's team was optimistic doctors at the Omsk hospital would release him. From midday local time on August 21, a German air ambulance was waiting at the local airport to take him to Berlin.
Then the medics said that "our Moscow colleagues" had insisted that Navalny stay until his condition was stable. Late that same day, the doctors reversed course -- and Navalny was flown out the following morning.
Some experts have suggested that Russian authorities planned to delay permission until it was likely that traces of Novichok had left Navalny's body. But they say proteins identifying Novichok would have remained in his body for weeks.
What authorities didn't know was that items recovered from Room 239 at the Xander Hotel were taken to Germany on the same medevac plane as Navalny. At least two subsequently tested positive for traces of Novichok, including a water bottle from the hotel room.
Just over a week after Navalny was wheeled into Berlin's Charité hospital, still in a coma, German Chancellor Angela Merkel declared that lab results showed "beyond a doubt" that the poisoning was "an attempted murder with nerve agent."
The European Union then imposed sanctions on several senior officials, including the director of the FSB, Aleksandr Bortnikov, saying he was "responsible for providing support to the persons who carried out or were involved in the poisoning" of Navalny.
CNN cannot confirm with certainty that it was the unit based at Akademika Vargi Street that poisoned Navalny with Novichok on the night of August 19, but its activities in July and August suggest that the European action against Bortnikov and other senior officials is not misplaced.
Both the Kremlin and the Russian security services have repeatedly denied any role in Navalny's poisoning. "To say that on the territory of Russia, there is production or stocks of military-grade poisons is of course disinformation," the head of Russia's foreign intelligence service, Sergei Naryshkin, said on September 15.
CNN approached both the Kremlin and the FSB about the investigation. Putin's spokesman declined to comment. No response was received from the FSB.
On Monday, CNN visited the home of Tayakin, who monitored the toxins team's communications while Navalny was in Siberia. When asked whether he was involved with the unit, he abruptly closed the door without comment.
Russian officials and media have advanced dozens of scenarios to explain Navalny's poisoning, suggesting it might have been done on the plane that took him to Germany. One of the favorite theories of state media is that Pevchikh was responsible, working for British intelligence in an effort to smear the Russian government. Pevchikh has dismissed the allegation.
On Friday, Putin told Russia's Human Rights Council that, while it was not necessary to open a criminal investigation every time someone nearly died, he had asked prosecutors to look into the Navalny case.
Russian analysts were looking at the materials available to them, Putin said, but were being blocked elsewhere.
"Our specialists are ready to travel abroad -- to France, Germany, and the Netherlands -- to see specialists who claim that poisonous warfare agents have been found there," Putin said. "Nobody invites us. We invited them to us. They don't come to us."
Navalny hopes the investigation by Bellingcat and CNN will lead to a raft of tough sanctions against the elite around Putin. So far he has been disappointed by the US response. "President Trump was asked about it and he said, let's talk about this later," Navalny told CNN with a shrug.
Under US law, should "persuasive information" emerge that a foreign country has used chemical weapons, the administration is obliged to apply a variety of penalties, including import and export sanctions.
Despite the risk, Navalny plans to return to Russia as soon as his doctors give him the all clear.
"I will go back and I will go back because I'm a Russian politician. I belong to this country," he said.
"I understand the whole operation. I would never give Putin such a gift."
This story was reported by Clarissa Ward, Tim Lister and Sebastian Shukla in southern Germany. Ward and Shukla also reported from Tomsk and Moscow. Lister and Shukla also reported from Vienna. CNN's Oscar Featherstone and Darya Tarasova contributed to this story.