Should the government arrest all people who are under surveillance for suspected terrorism (S-files)?

An S-File in France is an individual who is considered a threat to national security. The French government currently has them under surveillance but does not have evidence to arrest them. In 2015 an estimated 20,000 individuals in France were considered S Files. Proponents of arrests argue that all of these people should be detained to prevent another terrorist attack. Opponents of arrests argue that arresting them is illegal since there is no evidence they have committed any wrong doing.