1. Insofar as could be determined...
2. Insofar as I could determine...
If you don't want to express the agent, you can use #1. This may make the statement seem more impersonal, e.g.
3. Insofar as could be determined by electron microscopy, the immunosuppressive viruses were of endogenous origin.
This structure might be useful in a scientific or forensic context, for instance.
In #2, on the other hand, the agent is expressed. I would not say that #2 necessarily gave a "subjective" impression, though; "insofar as I could determine" is itself a cautious phrase, and might make a text seem "objective" to the reader.
Moreover, if we want to, we can express an agent in the #1 structure:
4. Insofar as could be determined by my initial investigations...
In other words, the "objectivity" of the statement would depend on the entire context!