Bob actually knows about this because he was still working at the FBI when this happened. That is why he knows James Burke and Thomas Spota were up to no good. But, Bob cannot reveal to the Captain that he knows this from the FBI.
He also learns that someone did find evidence in James Burke's car. A young man, Loeb, broke into James Burke's car and took a bag from the car. Turns out, there were some bad stuff in that bag-- obscene adult toys and tools and even some dirty clothes that had traces of victims' DNA. Yeah, that is how bold and shameless James Burke is. Keeping incriminating tools in the car, thinking that no one can arrest him.
"Sir," Bob stands up and looks at his Captain, "I think I know what you want me to do. You want me to find enough evidence to nail James Burke and Thomas Spota. But I think I have a better idea."
"What is it?"