Transcript for Taylor Swift's mom in court claims former DJ sexually assaulted singer
Taylor Swift appearing in court today facing the man she says groped her before one of her concerts. The two are locked in a multimillion-dollar lawsuit over the allegations. And today Taylor's mother took the stand to testify about her daughter's alleged trauma. Here's ABC's Clayton Sandell. Reporter: Taylor Swift is used to performing for thousands of screaming fans, as she did on her "Red" tour. ??? Ever, ever ??? but this time the only audience that matters is an eight-person jury. Now the legal bad blood between the pop star and former country radio dj David Mueller is heating up. David swift was back in a Denver courtroom for a third day, as seen in this sketch, facing the man she says groped her rear end at her concert four years ago. It's a legal he said/she said. He's suing her, believing she's responsible for getting him fired. She's countersuing, alleging assault and battery. I don't think this case is going to be made or broken based on the fact she's Taylor Swift. Some people like to punish celebrities. Some people are jealous of celebrities. It doesn't always help to be a celebrity in a high-profile case. Start at the front of the line. I don't know if I have enough for everybody. Reporter: The scene outside this Colorado federal courthouse felt like one of her concerts. I'm Taylor Swift's biggest fan. Reporter: Swifties lining up as early as 4:00 A.M. We're here from Utah, we drove through the night and then called off sick from work and then we're going to drive back right after the trial and be back at work tomorrow. Let me know if you want both or just the morning. Reporter: Some waiting five hours for a chance to grab one of only 32 seats in the courtroom in the orbit of one of the planet's biggest stars. Taylor has one of the most loyal fan bases in the entire world. Her fans came out in droves to support her. Across from the courtroom there is this adorable group of fans who have been making post-it notes supporting her saying "Haters gonna hate." ??? I'm just gonna shake, shake, shake, shake, shake it off ??? Reporter: The epic legal showdown for the "Shake it off" singer all began June 2nd, 2013. That's when Mueller then the morning host at Denver country radio station kygo took his girlfriend to a swift constant sxrt a brief meet and greet before the show, posing for this photo. Later leaked to TMZ. Now key evidence. This photograph is a really unusual piece of evidence. Usually, sides are fighting about which evidence they want the jury to look at. Here both sides say it's a good picture. Of course he says it shows that I'm not lifting her skirt, for example. But she says why would your hand be anywhere near my bottom. Reporter: Swift claims Mueller intentionally reached under her skirt and groped with his hand an intimate part of her body in an inappropriate manner. Mueller admits he touched swift's ribcage and arm, leaning in for that photo, but denied grabbing her back side. In her deposition swift says it was not an accident. It was completely intentional "And I've never been so sure of anything in my life." And neither is Taylor's mother Andrea who testified when she saw that photo "I knew something horribly wrong was going on," dramatically singling out Mueller today in the courtroom. "He sexually assaulted her. That guy." Seeing her mom on the stand, it really makes you realize that Taylor is just like any other girl. She has a mom who loves her and wants to protect her and only wants the best for her. Reporter: Andrea says team swift didn't report the incident to police because she didn't want it to define her daughter's life. You can understand why she might not want the publicity associated with making an allegation against a small-time radio disc jockey grabbing her on a photo line-up. Reporter: Yesterday on the stand Mueller took a grilling from swift's attorney about a secret recording he made with his bosses the day before he was fired. Most of that two-hour file vanished, Mueller claimed when he says his cell phone was accidentally destroyed and his lap fop top ruined by spilled coffee. "I wish we had the full recording," Mueller said. Swift's attorney countering "It will have shown whether you changed your story. Jury will never know." This case comes down to four basic questions. Her credibility, that picture, his credibility, and why did he destroy one of the critical pieces of evidence in this case. Reporter: Mueller is suing for damages that could total $3 million. But for swift it's not about money. She countersued for just $1. Anything more the jury awards her she says will go to charity. Taylor is showing women everywhere that whether you're rich or famous or just an everyday girl you can fight this and stand up for yourself. Reporter: The trial is expected to last nine days, where swift will take the stand with her own style. ??? We never go out of style ??? she says using her own struggle to help others. For "Nightline" I'm Clayton Sandell in Denver.
This transcript has been automatically generated and may not be 100% accurate.