Spanish officials say a suspicious envelope has been discovered at the US Embassy in Madrid and placed under police control.
The finding on Thursday came as police reported a wave of explosive packages being sent in Spain's capital, including one that ignited at the Ukrainian Embassy.
Police said other explosive devices concealed in postal packages were sent over the past two days to Spain's Defence Ministry, a European Union satellite centre in Spain and an arms factory that makes grenades sent to Ukraine.
The Spanish Interior Ministry said police detonated the envelope detected at the US Embassy. There were no reports of injuries.
Authorities also said that a similar explosive device was dispatched by regular post to Spanish Prime Minister Pedro Sanchez on November 24, but was disposed of by bomb squad experts.
The latest discoveries raise to five the number of explosive devices that were sent by post around the country over the past few days.
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The Russian embassy in Madrid on Thursday condemned the letter bombs, saying in a tweet that any threat or terrorist attack, especially those directed at diplomatic missions, are totally condemnable".
The package dispatched to the Ukrainian embassy and addressed to the ambassador slightly injured an employee when it burst into flames while he was handling it.
Ukrainian Foreign Minister Dmytro Kuleba quickly ordered that security be stepped up at all Ukrainian embassies abroad and has asked his Spanish counterpart for a speedy investigation.
Spain's National Court is investigating the incident as a terrorist act and extra security has been assigned to the Ukrainian embassy.
Police said all but one of the four letter bombs were disposed of. Interior Ministry State Secretary for Security Rafael Perez said police were using the one intact explosive device and its package that was sent to the air base to assist investigators.
He said an initial assessment was that all five packages were likely sent from within Spain.
Perez said that security has been stepped up at public buildings following the discovery of the package that was sent to Sanchez. This has now been extended to embassies, which already have extra security measures in place since the start of the Russian invasion of Ukraine in February.
The Defence Ministry package had been addressed to minister Margarita Robles, Perez said. Spain has contributed both military and humanitarian aid to Ukraine since the start of the Russian invasion.
The arms factory, located in the northeastern city of Zaragoza, makes grenade launchers that Spain has sent to Ukraine. The parcel was addressed to the factory's director.
A government official in Zaragoza said that both the arms factory and Ukrainian embassy packages had the same email address listed as the sender. No further details were given.
Officials said extra security forces have been deployed at the Torrejon de Ardoz base, just east of Madrid. They said the package was sent to the director of the base's European Union Satellite Centre.
The sending of small explosive devices in postal parcels is not uncommon in many countries. They were common occurrence for many years in Spain, especially during the most active years of the now defunct armed Basque group ETA.
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