The Ben Roethlisberger sexual assault investigation is complete ... except for one Big thing.

Milledgeville Police Chief Woodrow Blue tells TMZ they've completed their investigation -- including a follow-up interview with the alleged victim -- but they'd still like to button it up with a follow-up interview with Big Ben. Problem is -- Ben's not talking.

So assuming Ben exercises his right to remain silent, the Chief is still kosher with the case -- kinda sorta. We asked Chief Blue if he's satisfied, and he said, "Yeah, I guess."

Ben's lawyer isn't talking about why Ben isn't talking.

As for when a decision will be made on whether to prosecute the Pittsburgh Steelers QB, we're told the Georgia Bureau of Investigation will make that call ... "soon."