Help the detective finish his official report by choosing the correct narrative tense for each gap in the timeline of events.

When I finally breached the study, it was obvious that Lord Reginald had been shredding documents for at least an hour. However, by the time I crossed the room, he was sipping tea calmly in his armchair, insisting to my officers that he had just woken up from a nap.

When I finally breached the study, it was obvious that Lord Reginald had been shredding documents for at least an hour.

The Past Perfect Continuous (had been shredding) is required here because the action was ongoing for a duration ("for at least an hour") leading up to a specific moment in the past ("When I finally breached").

However, by the time I crossed the room, he was sipping tea calmly in his armchair...

The Past Continuous (was sipping) sets the background scene at the exact moment the detective crossed the room. It describes an action in progress at that specific past time.

...insisting to my officers that he had just woken up from a nap.

The Past Perfect Simple (had just woken) is used because the act of waking up was completed before he made the insistence in the past. The distractor has just woken is Present Perfect and does not backshift correctly into a past tense narrative.