Kate Upton's sexual assault allegations against Guess co-founder Paul Marciano is now facing another challenge as the potential witness refuses to cooperate.

Yu Tsai, the photographer Upton previously claimed to have witnessed the groping incident is not returning any communication. Reports stated that Yu has not been returning any of the phone calls and e-mails from the investigators.

Witness Gone AWOL

Investigators hired by Guess speculate that one of the reasons the star witness went radio silent is due to the fact that he had sent Marciano dozens of e-mails even prior to Upton's case.

One e-mail dated sometime in 2010 indicates that Kate asked for Marciano's phone number so she could extend her thanks for the Guess clothing she received. It also appeared in another e-mail that Yu apparently consulted Marciano for future photo shoots with Upton. Investigators think that Yu's silence could have been a signal of conflict of interest.

Upton joins the growing number of celebrities and entertainment staff who are filing sexual assault cases against their peers in the industry. The 25-year-old actress said that Marciano "grabbed her chest" and "forcibly kissed her" in a Guess lingerie event in 2010.

Upton further detailed that Marciano asked her to meet him, and as soon as she opened the door, the accused started playing with her chest. The actress said she was accompanied by Yu at the time of the incident, which pointed him as the primary witness of the case.

Smoke And Mirrors

Guess initially hired Marciano's law firm to conduct the investigation on allegations of sexual misconduct. Upton still refused to talk to the lawyers even if Guess had hired Glaser Weil, an independent law firm. The actress said the company is trying to execute a "smoke and mirrors" tactic.

"I'm truly disheartened to learn that Guess is treating this abuse of power and sexual misconduct (to multiple women) in such a manner," Upton shared on Instagram.

She also emphasized that hiring lawyers who have vested interest in the case will taint the investigation.

"To conduct an investigation like this seems like a stunt, and is a clear indication to me that Guess is not taking these claims seriously, nor are they committed to making changes at the highest levels," Upton added.

In February, Marciano announced that he is temporarily leaving his executive duties until results of the investigation are final. Since the time Upton revealed her story, Marciano has lost at least $52.8 million from his $12.7 million worth of Guess shares and options.

"I have pledged my full cooperation to the Company, and I have the utmost confidence in our CEO, Victor Herrero, to continue leading the Company during this time," Marciano said in a statement.

 Let’s make true change happen! #metoo #TRUEchange A post shared by  Kate Upton (@kateupton) on Feb 28, 2018 at 1:39pm PST