The regional police force of the Basque country has expressed its "dismay" after one of their officers died from a cardiac arrest following violence between supporters of Athletic Bilbao and Spartak Moscow on Thursday.
Trouble erupted near Athletic's San Mames stadium before the Europa League round of 32 second leg, with the two sets of fans attacking each other.
Television footage showed groups throwing objects towards both the police and the supporters of the other club before kick-off, with earlier violence resulting in a police officer taken to hospital with injuries.
During the violence just before the match, a flare thrown from a group of Russian fans landed close to an officer – identified as 51-year-old Inocencio Alonso Garcia – and he suffered a fatal cardiac arrest soon after, later being pronounced dead at the Basurto University hospital.
The Ertzaintza, the Basque police force, issued a statement on Friday.
It read: "The security counsellor, Estefania Beltran de Heredia, together with the rest of the heads of the Department and the Ertzaintza, wish to publicly express their dismay and deep indignation after the death of the agent of the Mobile Brigade, Inocencio Alonso Garcia.
"At the same time, we ask for maximum serenity when facing this difficult moment for the entire Basque police family.
"The department conveys its condolences to the families, to the Mobile Brigade and to the whole of the Ertzaintza and publicly thanks the numerous expressions of condolence received so far.
"Despite the speed of the transfer to the hospital, he had entered cardiac arrest. The preliminary report issued by the Basque Health Service - Osakidetza - establishes that the agent entered the Basurto hospital with cardiorespiratory arrest, not related to any type of trauma, and has died after more than an hour of attempted resuscitation."
The Ertzaintza also confirmed five arrests were made, while "several people" were identified beforehand with extendable batons, knuckledusters, bags of stones and knives.