Truck runs over 84 French people

[RobertE]

The trouble with that, of course, is that breaking people out of jail and trying to go fight in Syria are not terrorist acts, per se.

Boru

Apologies I made a few mistakes in the details. 

These are some important points better worded than how I put it that can be found on wikipedia. 

Chérif, also known as Abu Issen, was part of an informal gang that met in the Parc des Buttes Chaumont in Paris to perform military-style training exercises and sent would-be jihadists to fight for al-Qaeda in Iraq after the 2003 invasion.[94][95] Chérif was arrested at age 22 in January 2005 when he and another man were about to leave for , at the time a gateway for jihadists wishing to fight US troops in Iraq.
On 28 March 2008, Chérif was convicted of terrorism and sentenced to three years in prison, with 18 months suspended, for recruiting fighters for militant Islamist 's group in Iraq

French judicial documents state Amedy Coulibaly and Chérif Kouachi travelled with their wives in 2010 to central France to visit Djamel Beghal. In a police interview in 2010, Coulibaly identified Chérif as a friend he had met in prison and said they saw each other frequently.[101] In 2010, the Kouachi brothers were named in connection with a plot to break out from jail another Islamist, Smaïn Aït Ali Belkacem. For lack of evidence, they were not prosecuted.

So presumably there was evidence of them being part of terrorist organisations but not enough evidence of them plotting to break this person out of jail.  Still pretty important to note that they are convicted terrorists living in the capital.

Indeed, it is a question a large number of my colleagues ask each time. "Why don't they arrest them or detain them?" Well, it would be a good idea, but then should it be a large network, then the prison system would become ever more over-crowded than ever. Furthermore, they cannot really arrest someone because of suspicions; in fact they need evidence, and in spite of being on their lists, these people always get long enough to do whatever they want.