The independent Bellingcat research organization claims to have more information that the two men suspected in the poisoning of former Russian spy Sergei Skripal have links to Russian military intelligence, known as the GRU.
Bellingcat said on September 20 that a joint investigation with Business Insider "can confirm definitively" that the two suspects, Ruslan Boshirov and Aleksandr Petrov, have links to the GRU, "based on objective data and on discussions with confidential Russian sources familiar with the identity of at least one of the two persons."
On September 14, Bellingcat said it had reviewed Russian documents that indicated the two men had no records in the Russian resident database prior to 2009, a sign they may be working as operatives for the government.
"Crucially," Bellingcat added at the time, "at least one man's passport files contain various 'top-secret' markings which, according to at least two sources consulted by Bellingcat, are typically reserved for members of secret services or top state operatives."
In its latest report, Bellingcat said it and Business Insider obtained Petrov's and Boshirov's border-crossing data for several European and Asian countries. It said the men's names are believed to be aliases.
"Their globe-trotting, unpredictably meandering itinerary is at times reminiscent of characters out of [film series and television program] Mission Impossible, yet a focus on the countries of Western Europe is clearly visible," it said.
Bellingcat said a source in a Western European law enforcement agency informed it that the suspects had been previously arrested in the Netherlands, but "no information has been provided as to the time and context" of the arrests.
Bellingcat said it discovered there were just 26 intervening passport numbers between Petrov's document and the cover passport issued for Eduard Shishmakov, aka Shirokov, a former Russian military attache in Warsaw expelled by Poland in 2014 for espionage.
Shishmakov’s passport was issued in August 2016, the report said.
The finding suggests that the special authority that issued the passports had only granted 26 passports between April and August 2016, Bellingcat said.
It has been previously reported that the passport numbers of Boshirov and Petrov differed only three digits and that they held "Top Secret" and "Do Not Provide Information" markings.
The documents were allegedly issued by an authority normally reserved for intelligence officers and important government officials, it said.
Skripal and his daughter, Yulia, were found unconscious on March 4 on a bench in the southern English town of Salisbury. They were seriously ill but later made a full recovery after spending several weeks in a hospital.
British officials said the two were poisoned with Novichok, a military-grade chemical weapon that was developed in the Soviet Union, and blamed Russian President Vladimir Putin's government for the attack.
In June, a British citizen, Dawn Sturgess, died and her boyfriend, Charlie Rowley, fell ill when they stumbled across remnants of the poison in a town near Salisbury.
Britain on September 5 announced charges against the two Russian men as police issued photographs of the suspects.
The men acknowledged they were in Salisbury at the time, but claimed they were there as tourists.
Russia has denied any involvement in the poisoning.
With reporting by telegraph.co.uk and themoscowtimes.com