This article contains some more details... Its messy. The US refutes a claim by Spain that the sort of explosives used are similar to ones the ETA has used in the past, arguing that the ETA hasn't used those explosives in years, and they are widely available. However, they also claim that the organization which claimed responsibility has claimed responsibility in the past for things they were clearly not involved with, such as the power outage in the north east last summer. This raises doubts about their threat on the US. I've also read that the van is a little suspicious because the tape found in it is an educational tape, something under the head of a devout muslim. It could be an attempt at indirection. Also, Al Q usually doesn't declare responsibility for attacks so soon after they have occurred.

The fact that this UK group is nutzo and declares responsibility for all kinds of things does not mean Al Q is not responsible. Stratfor mentions another Al Q claim on the jihadunspun website, which since appears to have had its virtual host removed. (Good riddance to bad rubbage!)

If it does turn out this was the ETA it is a very dramatic change in their tactics, and really opens a new era of terrorism tactics.

Bombing Clues Point To Islamist Terrorists (washingtonpost.com)