Spanish police have arrested a 74-year-old man suspected of sending letter-bombs in November and December to the Ukrainian and US embassies, the prime minister's office and an arms manufacturer in Spain, the Interior Ministry says.
The man, a Spanish citizen, was detained in the northern town of Miranda del Ebro and police conducted searches of his home, the ministry said in a statement without providing further details.
Plain clothes police spent Wednesday morning searching his third-floor apartment in a blue-collar neighbourhood of low-rise cinder-block apartment buildings while armed officers guarded a cordon thrown up around the lane outside the property.
The suspect was understood to still be inside the house as the searches were conducted, witnesses told reporters at the scene.
The man used to work for the townhall of Vitoria, a large city nearby, before retiring in 2013, a city spokesperson told Reuters.
The investigation is still ongoing, Interior Minister Fernando Grande-Marlaska told reporters from Vitoria, declining to comment on reports a Russian militant group could be behind the attacks.
"We are working on all possibilities," Grande-Marlaska said.
"The investigation is very advanced and we can really be very satisfied."
A total of six parcels with explosives were sent to targets including Prime Minister Pedro Sanchez, government offices, a European Union satellite agency and the US embassy between November 24 and December 2.
Most were defused although an employee at the Ukrainian embassy was slightly injured when one of the devices ignited.
"All those facts had the same modus operandi," Grande-Marlaska told reporters.
A source close to the investigation had told Reuters in early December that all the parcels had been mailed from the city of Valladolid, a two-hour drive from Miranda del Ebro.