“After September 11, 2001, cooperation and coordination between the European police and security services naturally increased. Nonetheless, tensions remained: the French were especially annoyed by the breaking up by Belgian and Dutch authorities of two interlinked Al Qaeda cells in which cell members were arrested and weapons, identity documents and propaganda material seized on September 13 [2001]. This was because they had been trying to infiltrate the groups with informers to generate long-term intelligence.”
– Rohan Gunaratna, Inside al Qaeda, Page 127