A Russian women at work told me that the city was put on trial for the massacre were they? All I could see them proving was that city officials knew of the plan to execute the family , possibly the execution order and did nothing to stop it.
Bolshevik's had no reason to take the family prisoner none. They were powerless at that point.
There was still a war going on - the Reds v the Whites. The Whites were trying to rescue the Romanovs and were close by when they were executed. They wanted to rallying point for the White Army, which is what the Tsar and his family were as long as they were alive.
The Bolsheviks were tying up all their loose ends... although I'm not sure how they justified murdering Tsarina Alexandra's older sister Elisabeth - by then she'd been a nun for over ten years and had given away all her possessions to the poor... not exactly a threat in any sense of the word. She had royal blood, that's all.