Update: I just thought to look up んとする in the dictionary, which I had not done before, and it is listed as a set expression meaning "to be just about to". I don't know what the ん is grammatically; perhaps someone can explain?
ん is the n-euphonic change of a classical auxiliary verb for volition/presumption む. You can think it as 放たれようとしていた in modern Japanese. (Actually, this む is the etymology of う/よう, which is the conjugation suffix of the volitional form.)