Why didn't they use their existing powers such as an NSL to get more evidence against him or did they want him to commit some act so they could use it as the argument to increase their surveillance powers?
OK, but exactly what might that extra evidence be? Right up until he walked into the club and started shooting people he'd presumably been within the law.
In a country where gun ownership is 'normal' (wake up America; gun ownership isn't normal) the police/FBI have to second guess someone's intention; they can't just stake out everybody just on the off-chance they'll decide to go shoot up some place.
Biting the hand that feeds IT
