The letter bombs sent to embassies, politicians and an air base in Spain were homemade, Spanish media reports.
On Friday, Czech police said via Twitter that a shipment similar to the Spanish ones had been received by the Ukrainian consulate in Brno but did not contain a bomb.
In total, the letter bombs were sent to six different targets in Spain during Wednesday and Thursday. An employee of the Ukrainian embassy in Madrid suffered minor injuries when he opened the letter on Wednesday. No other injuries are known.