Actually, no. In order to do the more involved things, "physical observation, bugging rooms, and breaking into phones or computers", they have to get a warrant.
No need for that. They only have to get a warrant if they want to use the evidence in a court of law - most intelligence gathered by the secret services (which is what this is about, not about police investigations) never makes it to the court, and is not even intended for that purpose. Only when they want to actually go and catch someone they start to play by the books - that moment it's getting simple as they know everything already, just have to redo bits of their work the proper way.