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A Spanish policeman stands guard near the US embassy in Madrid, on December 1, 2022, after they have received a letter bomb, similar to one which went off at the Ukrainian embassy.
CNN  — 

US officials believe that Russian intelligence officers directed a Russian White supremacist group to carry out a letter-bombing campaign that rocked Madrid late last year, targeting the prime minister, the American and Ukrainian Embassies as well as the Spanish defense ministry, according to current and former US officials.

Spanish authorities have yet to make any arrests in connection with the attacks, which wounded one Ukrainian Embassy employee, but they were widely suspected at the time to be linked to Spain’s support for Kyiv.

Some details of how, exactly, the campaign was directed and carried out remain fuzzy, two US officials said. It’s not clear how much knowledge – if any – the Kremlin or Russian President Vladimir Putin himself had.

Still, US officials now believe that the attack was likely a warning shot to European governments which have rallied around Ukraine since Russia’s invasion in February of last year.

The New York Times first reported on the alleged involvement of Russian intelligence in the attacks.

A State Department spokesperson declined to comment “on matters involving leaked intelligence or active law enforcement investigations,” and referred to the Spanish government “for information related to their ongoing investigation.”

“We condemn all attempts by entities to harm and intimidate government officials and foreign embassies,” the spokesperson added.

As the war rages on – and particularly if Russia’s battlefield position deteriorates – US officials expect Russia to try to look for proxy groups it can work with to drive up fear of possible terrorist attacks carried out by Russian-backed groups in Europe and the Middle East, one US official explained.

The State Department designated the White supremacist group, the Russian Imperial Movement, as a global terror organization in 2020. The group is believed to have connections to Russian intelligence agencies and has been used as a proxy force before, current and former officials familiar with US intelligence told CNN. But those connections are murky, these people emphasized, in part because the US lacks good visibility inside RIM.

But the possibility that an organ of the Russian government – the military intelligence agency, the GRU – appears to have been involved in the attacks is likely to drive up pressure on the Biden administration to name Russia as a state sponsor of terrorism, according to one current and one former US official. The administration has so far been loathe to take such a step, despite pressure from key congressional officials, including former Speaker of the House Nancy Pelosi.

There are drawbacks to taking that step, one US official noted, in particular that it limits the administration’s ability to engage with Russia in areas where it might want to.

The White supremacist group, RIM, has associates across Europe and operates military-style training centers within Russia but is not formally affiliated with the Russian government. But, one former US official said, “There’s no question that RIM operates in Russia because it’s allowed to operate in Russia.”

The GRU, meanwhile, has carried out increasingly bold operations across Europe and beyond, including assassination attempts. It is also believed to have offered bounties to Taliban-linked militants for killing US troops in Afghanistan, although in that instance, too, the intelligence reporting remained murky, and the Kremlin’s involvement was unclear.

CNN’s Jennifer Hansler contributed reporting.