I don't think it matters who said those words “ we know he did it, we just need to convince a jury”, either the investigator or the prosecutor.
Of course an investigator needs enough evidence to be able to convince a jury that the known perpetrator is guilty. They know that they can't hand a flimsy case over to a prosecutor, and a prosecutor will not accept a flimsy case.
The investigators and the prosecutor work together when amassing/compiling enough evidence to convince a jury.
I was reading yesterday about 'hold out' jurors, and the push to allow majority decisions - as opposed to unanimous decisions - of jurors. Sufficient and good, strong evidence is necessary ... even when the perpetrator is 'known' to have committed the crime.
There is no room for reasonable doubt, that a good defence lawyer can often easily create, in a trial.
Majority report