I'm not sure what you mean. She's a criminal profiler. She looks at the cadaver dogs' evidence, the evidence that someone broke into the apartment ( or lack thereof), the possible locations for the concealment of Madeleine's body. How could you interview witnesses who don't come forward?
The McCanns have been absolved of any responsibility into the disappearance of MM, other than leaving her unsupervised in the first place

Surely if you are carrying out an investigation into the disappearance of a child, or even an adult, the first port of call is to take a close look at those who were known to have been in close proximity around the time of the disappearance?