The prosecutors, along with Mueller, knew Russian collusion wasn't real but continued on with the investigation anyways ... essentially looking for process crimes to get anybody in the Trump orbit.
"According to a separate report on serial abuses committed by the Department of Justice (DOJ) during its investigation of Trump, the DOJ inspector general found that the government knew long before Mueller was even appointed that there was no evidence of any collusion."
https://thefederalist.com/2020/02/11/mueller-prosecutors-may-have-lied-to-doj-about-stone-prison-sentence-recommendation/