The Guess investigation into Kate Upton's sexual harassment allegations against co-founder Paul Marciano has hit another roadblock -- Kate's witness is refusing to cooperate.
As we reported ... Kate has already said she won't talk to the lawyers investigating for Guess. She's pissed Guess initially hired Marciano's personal law firm for the probe, and even though Guess has now brought in a new independent firm, Glaser Weil, Kate's still pissed about what she calls "smoke and mirrors" tactics.
Our sources say photographer Yu Tsai -- the man Kate claims witnessed Marciano groping her -- has also gone radio silent. We're told Yu hasn't returned multiple phone calls and emails asking him to talk to investigators for Guess.
One possible reason for Yu going AWOL could be the dozens of emails he's sent Marciano over the years -- including one in July 2010, immediately after Kate alleges Marciano groped her. In that email, which we've seen, Yu says Kate asked him for Marciano's phone number so she could thank him for some Guess clothing she received.
In other emails, he consults with Marciano about future shoots with Kate and gushes about getting to work for Guess. It certainly appears he admires the Guess co-founder.
You'll recall, Kate said Yu was there when Marciano allegedly groped her thighs and breasts, and he's backed up her version of events. She said Yu was her buffer, present when she had a meeting or photo shoot Marciano attended.