"And agent is not the same as informer"

The difference is not really clear in reality and can get blurry quickly. Informants can also be ordered to do more active things to get information - or spread missinformation, or be ordered to do other active things. Simple things like luring a person to a party, or using their influence of position to prevent or make something happen (punish/promote a student/worker, make a certain result go away, ..).

So the majority were likely passive informants with the job of writing reports, but not all of them. And to me it rather would matter if they took the job voluntarily or were forced to do it.