A Russian agent linked to a Kremlin hit squad tailed a critic of president Vladimir Putin before he was murdered, a new investigation reveals.
And in a fresh twist, the commander suspected of the murder has allegedly been spotted in the besieged Ukrainian city of Mariupol.
Putin rival Boris Nemtsov was murdered in 2015 after speaking out against Russia's first military action in Ukraine in 2014.
Nemtsov had reportedly been shadowed by an agent who had links to a hit squad, and was later shot dead only metres away from the Kremlin.
Seven years on and a new investigation from Bellingcat and the Insider - in partnership with the BBC - has revealed the main suspect in his killing is a former operative for the FSB, the Russian security service, Valery Sukharev.
Sukharev reportedly tailed Nemtsov on at least 13 journeys during his time at the FSB.
In a new development, prime murder suspect Ruslan Geremeyev has now been spotted in the Ukrainian city of Mariupol, which is currently being mercilessly besieged by Russian forces.
The intel was obtained from a database called Magistral, which is used by the FSB.
Corrupt officials reportedly leaked the information, revealed through flight and train reservations, to investigators.
The Magistral database tracks movements of both agents and the people they are following.
Executive director of Bellingcat, Christo Grozev, told the BBC that corruption within Russia meant that a resource like Magistral would be a 'double-edged sword' because 'it allows people like us to actually go and tail these same spies, these same FSB officers'.
Five men of Chechen origin were arrested and jailed in the wake of Nemtsov's murder.
But this did not satisfy Nemtsov's family, who pushed for investigators to look into possible involvement of Chechen leader Ramzan Kadyrov.
Geremeyev had served as the commander of the unit of one of the individuals who was jailed, the Nemtsov family's lawyer Vadim Prokhorov also called for him to face questioning over Nemtsov's death.
Prokhorov has even named Geremeyev as the person behind the killing, and he fled to Dubai after being declared a suspect so never faced questioning.
The US treasury subsequently imposed sanctions on Geremeyev over the killing, as well as allegations relating to torture.
Geremeyev has now re-emerged in a photo taken in the besieged Ukrainian city of Mariupol alongside armed companions and shared on Telegram.
The 43-year-old is said to be part of the Chechen contingent fighting there, as reported by the Times.
Ukrainian officials have claimed that as much as 90% of the beleaguered city has been heavily damaged in a month of Russian bombardment.
It was home to 400,000 people before the Russian invasion.
The image of Geremeyev shows a Chechen flag flying behind the group, with the face of Ramzan Kadyrov displayed on it.
Kadyrov has also described Geremeyev as 'dear brother', and said that he 'always gets the job done'.
He added that: "He was assigned the most difficult area of Mariupol, but in this task the commander has acquitted himself perfectly.”
