Thank you, Toritoribe-san and benten-san.
But my textbook says verb (volitional form) + としていた／としている means the action is about to happen (but has not happened yet). For example, 水が浴槽からあふれようとしている。(the water has not yet spilled over the sink) So whether the pattern verb (volitional form) + としていた／としている can mean the action has already happened depends on the context?
Or, it depends on what the verb expresses. For instance, 脱がそうとしていた can refer to a situation where the subject already touched his underwear and tried to pull it down (but didn't pull it down yet). That's what I said by "the subject already started the action".