how did Detective Keith Frazier & Detective Bill interrogate hostages?

Because the events are told in a nonlinear way:

The story is told in an unusual, non linear manner, with flash-forwards to interviews with people involved in the robbery a la Warren Beatty’s “Reds” (1981).

Wikipedia explains:

Nonlinear narrative, disjointed narrative or disrupted narrative is a narrative technique, sometimes used in literature, film, hypertext websites and other narratives, where events are portrayed, for example out of chronological order, or in other ways where the narrative does not follow the direct causality pattern of the events featured, such as parallel distinctive plot lines, dream immersions or narrating another story inside the main plot-line. It is often used to mimic the structure and recall of human memory, but has been applied for other reasons as well.