Still not sure exactly what groups were involved, but prior to the Atletico Madrid-Deportivo La Coruña match there was a battle between groups of fans from various ultra groups--latest information is the Riazor Blues (Depor), Frente Atletico, Bukaneros (Rayo Vallecano) and a group from Alcorcón, Alcor Hooligans. One report is these groups were tipped off by an Atleti supporters' group where the "Blues" would be, and so prepared an ambush.
What is clear is that as a result of the fighting, a 43 year-old Depor supporter, presumably a Riazor Blues member, has died of a cardiac arrest after being rendered unconscious and was thrown from a bridge into the Manzanares River near the stadium. 11 others were injured.
At least two people have been arrested and numerous others identified as being involved in the violence, and there will be a meeting of the Sports Council in Madrid on Monday to discuss what happened. Supporters' groups such as these are very visible in many grounds, provide a lot of good atmosphere and noise, and for the most part are well-behaved. However, some, especially at the larger clubs, have elements who embrace violence and right-wing agendas, just as are found in ultra groups in many other European countries.
UPDATE-- The dead man has been identified as a member of the "Los Suaves" group, described as the 'most radical of the Deportivo ultras". Depor have issued condolences to his family and friends, and have said on their webpage they are not yet clear what occurred. A banner was shown after the match saying "our passion is not worth a life" and there were also photos of supporters exchanging scarves