This is true, if we look at their past track record. Don't think for one minute that they aren't monitoring the local population, or websites like this one. The very notion of terror is the unknown. Who are they, where are they, and when will they strike again?
Sometimes it is more useful to not claim responsibility. At least immediately. I'm not saying that I believe that this refinery accident was a bona fide act of terrorism, but it sure has our tongues wagging, doesn't it?
There is only ever one reason not to claim responsibility and that is when the operation has in the eyes of the group responsible gone wrong, the wrong people were killed, this happened very rarely in North Ireland, one case a pregnant women with another baby in a pram was killed causing wide spread revulsion.
That is not the case here, this is a economic blow.
All operations carried out by terrorist are governed by political, propaganda as well as the destruction caused.
It is to tell the target population you are not safe, we can strike anyhow anywhere, and striking and not telling goes against these tenets, tenets that have been avidly followed by Islamic groups, maximum destruction maximum publicity.
These are the most camera loving terrorists we have met.