Where all the percipient witnesses in a case have testified in a manner unsatisfactory to the government's prosecution, under Federal Rule of Evidence 702, which requires an expert's testimony to "help the trier of fact to understand the evidence or to determine a fact in issue," may the government be allowed to introduce law enforcement 'experts' to provide testimony that contradicts its own percipient witnesses in order to make its case? Should these officers have any limitations placed on their testimony?

Whether law enforcement expert testimony contradicting percipient witnesses satisfies Federal Rule of Evidence 702