They've announced the modus operandi of the bombing operations: what the killers did was simply get on a train, unobtrusively leave a pre-set backpack, and leave through another door before the train even left the station. This "rudimentary" technique, as they called it, worked perfectly, and it fits in with ETA capabilities now: three or four kids could have pulled this off, assuming the bombs were pre-manufactured for them in some ETA hideout near Hernani or whatever. There's absolutely no reason to suspect this was anything but an ETA job. It might be interesting to find out exactly what connection ETA has with international terrorist gangs like Al Qaeda, though.