There are two reasons why a terrorist group haven't claimed it yet, that I can think of. I do know a Chinese group claimed it but I believe this was discounted.
- It wasn't a terrorist/hijack at all. It was pilot suicide.
- This is just the beginning and the main event is still to come,
Whether that is re-using the plane, although I still struggle to believe they have successfully got away with landing it somewhere, or whether the plane was never intended to be found or used, i.e. it is at the bottom of the ocean, and the ultimate aim is to cause panic and chaos, perhaps by doing it again, as I mentioned before. In any case, there are plenty of people who are prepared to die for their cause and sometimes that means taking as many other people down with them. That is what is truly frightening about all this.