Two Russian men suspected of carrying out the 2018 Salisbury poisonings are being linked to an explosion at an arms depot in the Czech Republic. Evidence links the 2014 explosion, and an attempted poisoning in Bulgaria, to a unit of Russian military intelligence - the GRU - the BBC has learnt.European intelligence agencies believe the GRU's Unit 29155 is tasked with sabotage, subversion and assassination. The Russian government said the claims were unfounded and absurd. Czech authorities say they are expelling 18 Russian diplomats believed to be intelligence operatives in retaliation for the explosion, which killed two people.Czech Prime Minister Andrej Babis said the country had to react to revelations tying the blast to the GRU. The country will inform Nato and European Union allies about its suspicions, and will discuss the matter at an EU foreign ministers' meeting on Monday, its acting Foreign Minister Jan Hamacek said. The Russian foreign ministry said it would "take retaliatory measures that will force the authors of this provocation to fully understand their responsibility for destroying the foundation of normal ties between our countries". But painstaking detective work by Czech authorities has pointed the finger at Moscow - and Unit 29155 of the GRU.In the wake of the Salisbury poisoning, European security services have been investigating a series of previously unexplained events. For Czech police, that included the October 2014 explosion. A crucial find, sources close to the investigation have told the BBC, was an email sent to Imex Group, the company which operated the depot. It claimed to come from the National Guard of Tajikistan. It asked for two men to be given access to the site for an inspection visit. Scans of their passports were attached. The men were said to be Ruslan Tabarov from Tajikistan and Nicolaj Popa, a Moldovan citizen. The pictures on the passports match those of the two men accused by Britain of the Salisbury poisoning.