Al Qaeda lacks the resources and manpower to carry out more than one such attack a year. Putting Europe on high alert is closing the barn door after the horses are gone.
Reportedly one of the blasts was caused by a suicide bomber. There seem to be many volunteers for martyrdom in Islam so I wouldn't be too sure there aren't others waiting to make their move. In fact, if other cells are planning attacks I would expect them to occur quickly before the authorities can get a handle on this.