Two Russian men secretly working for Ukraine's intelligence services have given rare accounts of operations they claim to have been involved with, that have targeted military assets and personnel inside Russia.
One of the men, whom we are calling Ivan, claims to have played a role from Russia in Ukrainian drone attacks on strategic bombers and munitions factories on his home soil.
He features in a BBC World Service documentary about the shadowy world of Russian saboteurs.
The documentary, by independent Polish journalist Marcin Mamon, also features a man we are calling Foma, a Russian living in exile in Poland.
Foma runs a network of undercover volunteers in Russia, that surveils high-level officers that the GUR - Ukraine's military intelligence service - is considering targeting, he says. He stresses they are never involved in the attacks themselves.
He was also tight-lipped about who they were surveilling, but after Lt Gen Fanil Sarvarov was assassinated in December last year in Moscow, Foma admitted he had provided the Ukrainians with intelligence on him.
There have been previous media reports of Russian sabotage groups, but it is rare to get access to someone operating on the ground, like Ivan, explaining alleged activities.
The GUR has confirmed that Ivan and Foma are both working for the agency. But it also has acknowledged that it cannot know the full picture. When Ivan's handler was pressed on whether he could be trusted, the handler replied: "I'm not sure of anyone, except myself."
And both the GUR and the men are cautious about how much detail they share, making it impossible to verify the activities the men claim to have carried out.
The broad tactics Ivan and Foma describe, however, have been acknowledged by the Ukrainian authorities. Kyiv has spoken of its strategy of co-ordinated drone attacks on what it claims are legitimate targets inside Russia, and has acknowledged it uses secret agents for this.
There have also been several political assassinations in Russia since the start of the war. As a matter of policy, Ukraine never officially admits or claims responsibility for targeted attacks, though a Ukrainian intelligence officer has previously admitted to the BBC it was behind the killing of senior military figure Lt Gen Igor Kirillov in December 2024.
It is difficult to confirm how many Russians are doing this kind of work for Ukraine. According to Foma, they number about 300-500, though this is impossible to verify.
"You will get a file... from me or a colleague… with the proposed target. The file will contain [the target's] photo, his data, his details, date of birth, age, and a description of him. There'll be information on his car, if he has one. When you go to the target, don't take a phone," he told his volunteers.
Foma was once part of the Russian elite, so he claims to have a unique set of contacts and constant flow of insider information.
He showed Mamon on his laptop how he mapped military targets.
"This shows a database... the location of storage warehouses, and military units. Where generals live… Where the pilots of strategic bombers who shell Ukraine live.
"In order to break the system, you need to understand how it works. Then you can aim for their weak points."
He said there were about "three dozen" people in his network split between Russia, Ukraine and elsewhere in Europe, with the contingent outside Russia providing logistical support, such as tracking targets' digital footprints.
Foma stressed that the GUR suggests the targets to be surveilled, but that he and his team check for themselves that these individuals should be considered military criminals.
"As a rule… we also check the data [on that person]… Only after that we say whether we work or not."
But he argued that his team's actions were justified.
"I want people who do terrible things on behalf of Putin and Russia in Ukraine… to understand that they'll have to pay for it. I want them to get scared."
Foma stressed that he only passed on intelligence to the Ukrainians, and his network did not carry out any assassinations themselves.
Of the assassination of Gen Sarvarov, killed in December 2025, he later texted Mamon to say: "Keeping track of him took quite an effort. He lived alternately with his mistress and his colleague. Covered his tracks and constantly changed his schedule."
But Foma did not share any plans with Mamon and the BBC in advance.
Ivan, however, was tight-lipped about the work he had been involved with, even after the event.
Foma did his own digging on Ivan.
After checking with his military intelligence contacts in both Ukraine and Russia, he thought it likely that Ivan had been involved in a plan to strike Russia's largest military drone factory at Yelabuga in June last year, external. One person was reported to have been killed in the incident.
Ivan stressed that the aim is always to try to avoid harming civilians.
Operations have clearly been risky for Ivan too.
"The most serious case was when my car was stopped for inspection and I had the handmade explosives on me. The risk was very high - however as you can see, I was not detected," he said.
To mitigate the risk of being compromised, "there is no clear structure" to the work, he said, adding he had never met and did not know the identity of others he had been working alongside.
Foma has the same rule for his operatives.
"As soon as you start gathering people in groups, in general chats… leaving a mark on the internet, with a high probability this whole group will be quickly detained or destroyed," he said.
Ivan stressed there were few people left in Russia suitable for this kind of work.
"Most of the ideological people have either left the country or are already behind bars," he said.
Many Russians involved in sabotage work on their native soil are therefore either oblivious - for example lorry drivers unaware that their cargoes have been compromised - or are individuals who have been paid, the BBC understands.
Ivan, on the other hand, says he is doing this work for ideological reasons - he used to be a member of the political opposition. As such, his movements would be expected to be monitored closely by the Russian authorities.
So it was a surprise that he managed to frequently leave Russia and return.
Mamon persuaded Ivan to drive him into the Georgian desert, where they could speak freely - and asked him how the Ukrainians could trust him.
"You go to a third country… where [Ukraine's] agents meet you… and conduct lie detector tests," Ivan told him.
Mamon asked Ivan to put him in touch with one of his GUR handlers in Ukraine. This man immediately rang Ivan on speaker phone - apparently demonstrating that Ivan worked for him. They spoke in code, referring to "repairs at the school".
But later, the handler admitted he could not rule out the possibility Ivan might work for the Russians too, either voluntarily or under duress.
Foma thinks the fact that Ivan can travel freely does not necessarily constitute a red flag.
"There are all sorts of holes in [the country's governance], and if you know them, or are just lucky, then you can jump through the holes… and live in the system. Like a mouse in a piece of cheese," he told Mamon.
Perhaps more compellingly, if Ivan had also been spying for FSB, it would seem likely that he would share what he knew of Ukraine's attack plans with the Russian agency. More than a year before a devastating strike by more than 100 Ukrainian drones, Ivan had been telling Marcin there would be a major attack on strategic bombers on military airfields across Russia, though he never said when or where.
Marcin asked Ivan how he coped with the work.
"Fear is always present," he said. "It follows me everywhere. Even if you don't do anything, you're still afraid. This is Russia."
But he was optimistic that eventually his life would change.
"Putin won't live forever," he said.
