ETA normally targets politicians, so this doesn't sound like them either, unless, as I was wondering last week, someone in ETA has allied with an Islamist terrorist cell in the same way that IRA members - ostensibly Catholic - eventially allied with such Islamist cells as Hamas, PLFP and Hezballah.
It could be some group auditioning to become a part of the al Qaeda organization, which I believe was the case with the Turkey attacks. It's pretty common for gangs to require potential members to go murder people as proof of their loyalty.