According to various sources, the terrorist group arrested in Moroccan yesterday was responsible for the terror alarm in December, 2007 in Brussels.
This was the real reason for the alarm, which was announced December 21st and which caused the New Year's fireworks display to be canceled.
Speculation at the time connected the alarm to a group accused of plotting to free Nizar Trabelsi.
Source: HLN (Dutch)
See also: Brussels: Continuing terror threat