The Federal Prosecutor’s Office (BA) revealed that it had been separately investigating the pair for a year before they were arrested.The Skripals were poisoned in Salisbury on March 4.Novichok samples used in the attack were tested at the UK defence laboratory in Porton Down before being collected by inspectors from the Organisation for the Prohibition of Chemical Weapons (OPCW) on March 19.The inspectors also collected blood samples from Mr Skripal and his daughter Yulia, who at the time remained in a critical condition under heavy sedation in hospital, to conduct their own analysis. Sergei and Yulia Skripal Want the best of The Telegraph direct to your email and WhatsApp? Sign up to our free twice-daily Front Page newsletter and new audio briefings. Two Russian spies were arrested in the Netherlands in the immediate aftermath of the Salisbury poisonings as they allegedly attempted to hack into the computers of the Swiss laboratory analysing the Soviet-developed Novichok, it has emerged.The pair were detained in the Hague and extradited back to Russia following an intelligence operation run in conjunction with British, Dutch and Swiss agents.Their arrest in March is said to have been directly linked to the world-renowned Spiez laboratory near Bern that tested the military-grade nerve agent used in the attempted assassination of former Russia spy Sergei Skripal.Several sources told Swiss newspapers Tages-Anzeiger and the Tribune de Genève that the pair were arrested on suspicion of preparing to target the lab.They reportedly had equipment in their possession which would allow them to break into its computer system.The arrests throw a further spotlight on the two countries to which Mr Skripal’s would-be assassins travelled several times in the months leading up to the attack, increasing suspicions that they could be key locations for Russian agents embroiled in the plot. Flight records obtained by the Telegraph show that Alexander Petrov and Ruslan Boshirov made at least six separate trips to Geneva between November 2017 and February this year. The Spiez laboratory subsequently confirmed British assertions that the Skripals had been targeted with Novichok. The laboratory has also been investigating poison gas attacks by the Syrian regime, which is backed by the Kremlin. Ruslan Boshirov, left, and Alexander Petrov give an interview to the RT news channelCredit:TASS via Getty Images Alexander Petrov and Ruslan Boshirov captured on CCTV in SalisburyCredit:UPI/Barcroft Images They also travelled in and out of Amsterdam, both separately and together.Petrov and Boshirov this week insisted in a widely ridiculed television interview that they had come to Salisbury as tourists to visit its cathedral, “famous for its 123-metre spire and its clock”.Isabelle Graber, head of communications at the Swiss intelligence service, the FIS, confirmed that the two unidentified Russians, not thought to be Petrov and Boshirov, had been arrested earlier this year in The Hague.She told the Telegraph: “The Swiss authorities are aware of the case of suspected Russian spies discovered in The Hague and expelled from the same place.“The FIS participated actively in this operation together with its Dutch and British partners.” In April, Sergei Lavrov, the Russian foreign minister, claimed the lab had found the Western-made BZ nerve agent in the Salisbury samples and said the OPCW had questions to answer. He did not disclose the source of his information.The Swiss facility vehemently denied the claims, revealing that BZ was only used in the lab as a counter sample to Novichok.Swiss authorities revealed in July that the lab had been targeted by hackers believed to be linked to the Russian government, although it is not known if the expulsion of the two spies from the Netherlands was linked.The March arrests were not made public at the time although at the end of the month, Mark Rutte, the Dutch Prime Minister, announced the expulsion of two diplomats who worked at the Russian Embassy in The Hague, saying that “Holland had had enough”.