When asked how, the photographer said: "I saw his hand on her butt".
Taylor Swift has testified in her trial accusing a radio host of groping her that "he grabbed my ass underneath my skirt" when the two posed for a photo together before her 2013 performance in Denver.
"He sexually assaulted my daughter, right there, that guy", Andrea Swift said as she pointed at Mueller.
"This is what happened, it happened to me, I know it was him", Swift said, adding that she had no reaction to learning that Mueller, then 51, had been fired over the incident.
The photographer who took pictures at the meet and greet told the court she thought Swift had made a amusing move during the photo, at first believing she had tripped. "It was a very shocking thing".
He then sued the star, her mother and his former employer, claiming he was "falsely accused", seeking $3.8 million (AUD) in damages. Stephanie Simbeck said she is sure Mueller grabbed Swift's rear end but couldn't tell whether his hand went under her skirt.
"It was just destroying her", the elder Swift testified. She said it was clear to her that Swift "was trying to get away" from Mueller.
Mueller sued Swift after her team told his bosses at a music station that he reached under her dress and touched her backside before a 2013 concert in Denver. She declined to speculate how long the incident lasted.
Swift's appearance Thursday was about making the statement, to herself and the world, that whatever it was that happened was not her fault.
The station's program manager Herschel Coomer - known in the industry by the pseudonym Eddie Haskell - testified that Mueller was adamant he hadn't done anything improper.
The 27-year-old's mother Andrea took the stand and tearfully recalled her daughter telling her what had happened.
"I know my palm was facing down".
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In addition, the Squamish First Nation has gone to court to challenge the adequacy of the consultation process by the province.
While looking at a critical photo allegedly taken at the moment of the grope, Mueller's attorney asked Swift why the front of her skirt did not appear to be lifted. "I never have said anything different".
However, Gabriel McFarland, Mueller's attorney, said the photo did not show the inappropriate under-skirt touching that Swift has alleged.
Mueller testified on Tuesday that he may have made incidental physical contact with Swift, perhaps brushing her arm or torso.
Hershel Coomer, Mueller's supervisor at the radio station, said that before the night of the concert he had no desire to fire Mueller.
Robert Call, Mueller's boss at KYGO and the man who fired him, later told the court the photo indicated to him that Mueller's hand was in an inappropriate place and that this was a large reason for his dismissal.
Swift said she was stunned and did not say anything to Mueller or halt the meet-and-greet after he left because she did not want to disappoint several dozen people waiting in line for photos with her.
Swift, as captured by a court sketch artist, during the trial.
Why didn't Swift report the alleged groping immediately?
Mueller also hit the stand on Wednesday, and was cross-examined by Swift's attorney, Doug Baldridge, who attacked the radio host's claims that Swift's accusations had "rendered him unemployable".
Andrea Swift said she wanted to keep the encounter private because she didn't want it to define her daughter's life.
Mueller's suit doesn't seek a specific monetary amount, but an expert retained by the ex-radio host determined that almost $3 million was a fair compensation for damages.
Swift countersued Mueller, claiming sexual assault. She says she wants to be an example to other assault victims.