Brahim Abdeslam and Salah Abdeslam
The report, presented to Belgian lawmakers and seen by a number of publications, says the police had information on the brothers' radicalization, their relation to the would-be mastermind of Paris attacks Abdelhamid Abaaoud, and their apparent intention to perpetrate some kind of a terror act since at least the middle of 2014.
Additionally, the police found out in January 2015 that both Salah and Brahim Abdelslam intended to go to Syria, according to Le Soir newspaper. They had been questioned in relation to this. However, when the Belgian prosecutor requested to wiretap the brother's phone calls, the idea was reportedly turned down due to "lack of resources."
Comment: Turned down why? Similar events happened in the U.S. in 2000/2001 when two of the alleged 9/11 hijackers traveled to the States (al-Mihdhar and al-Hazmi). The CIA's Alec Station knew they had ties to al-Qaeda (including the 1998 African embassy bombings and the attack on the USS Cole), yet blocked agents from passing information about them to the FBI when they entered the U.S. The most likely reason? They were already being surveilled, most likely by a friendly foreign intelligence service, or the CIA itself. FBI surveillance would have messed up their plans, which were deliberately allowed to continue, or worse, directly facilitated by CIA.
Comment: For further discussion of this depressing phenomenon, see:
A sign of the times: Depressing survey results show how extremely stupid America has become