I also just had a look at the law this administrative extension extends, SR780.1 (http://www.admin.ch/ch/d/sr/780_1/index.html, unfortunatley I wasn't able to find an English translation)
(I'm no lawyer and not a native english speaker so my translation might be incorrect but you'll get the picture)
To be able to command surveillance the following factors have to be fulfilled:
a.
Certain factors justify the urgent suspicion, the person to be surveyed has commited one of the crimes listed in paragraph 2 or 3 or was part of such a crime.
b.
The gravity of such a crime justifies the surveillance
c.
Other tries at investigating have remained unsuccessful, or the Investigations without the surveillance would be futile or unjustifiably hindered.
In paragraph 2 and 3 follows a list of the crimes that justify a surveillance.
Especially interesting is Article 3: Prerequisits (http://www.admin.ch/ch/d/sr/780_1/a3.html):
(I'm no lawyer and not a native english speaker so my translation might be incorrect but you'll get the picture)
In paragraph 2 and 3 follows a list of the crimes that justify a surveillance.