A lot has been made of this, with people pointing to this as proof that the prosecutors have a suspect for Maura's murder but not enough evidence to convict. But I do not believe this is what he meant. Here's a snippet taken during his cross examination by Fred's lawyer.
Question: Based on your involvement with this investigation and your knowledge of the file, do you have an opinion as to whether or not it is more likely than not that this investigation may lead to criminal charges?
Answer: I do.
Q. What is your opinion?
A. I would have to say it's more likely.
Q. That it will lead to criminal charges?
Later, Strelzin goes on to say:
I could give a percentage of what I think that likelihood is, but I acknowledge that there's also a likelihood that this could simply be a missing person's case that doesn't have criminal overtones.
In other testimony, it is revealed that the investigation kicked up info on criminal activity that was possibly tangential to Maura's disappearance - an arson, etc.
So what does he mean by the 75% conviction? I believe he's talking about things other than murder. That 75% chance of a conviction could refer to things like identity theft, money laundering, fraud, assault, etc. But none of it is able to be pursued until Maura either steps forward or her body is found.
But don't assume the 75% chance refers to murder.