Bertram, who was 31 and a newcomer in Hollywood at the time of the incident, told The Hollywood Reporter that the alleged assault happened during the filming for Allison Anders directed "Gas Food Lodging" in Deming, New Mexico.
She narrated that the "Prison Break" star Knepper sexually assaulted her when she went to his trailer to drop some clothes.
Bertram said Knepper "jumped up", reached under her dress and grabbed her crotch. He then shoved her against the wall.
Knepper, 58, who has starred in films like "Transporter 3", "Hunger Games: Mockingjay Part 1" and plays the role of Theodore 'T-Bag' Bagwell on hit TV series "Prison Break", has denied the allegations.
"Over the past few weeks, my wife and I have discussed the pain women have experienced and the bravery they have shown in coming forward," the actor said in a statement posted on his official Instagram page.
"I am shocked and devastated to be falsely accused of violence against a woman. That's just not who I am. To those who have reached out today with words of support and encouragement, I thank you from the bottom of my heart," Knepper added.