Just after Valentine’s Day in 2019, the musician FKA twigs was in a car speeding toward Los Angeles. At the wheel was her boyfriend, the actor Shia LaBeouf. He was driving recklessly, she said in a lawsuit filed on Friday, removing his seatbelt and threatening to crash unless she professed her love for him.

They were returning from the desert, where Mr. LaBeouf, the star of “Transformers,” had raged at her throughout the trip, FKA twigs said in the lawsuit, once waking her up in the middle of the night, choking her. After she begged to be let out of the car, she said he pulled over at a gas station and she took her bags from the trunk. But Mr. LaBeouf followed, and assaulted her, throwing her against the car while screaming in her face, according to the suit. He then forced her back in the car.

The gas station incident is at the heart of the lawsuit that says Mr. LaBeouf, 34, abused FKA twigs physically, emotionally and mentally many times in a relationship that lasted just short of a year. Her aim in coming forward, she said in an interview, was to explain how even a critically acclaimed artist with money, a home and a strong network of supporters could be caught in such a cycle.

“I’d like to be able to raise awareness on the tactics that abusers use to control you and take away your agency,” FKA twigs, 32, born Tahliah Debrett Barnett, said.

Mr. LaBeouf responded Thursday to the concerns raised by Ms. Barnett, and a second former girlfriend who has accused him of abusive behavior, in an email that broadly addressed his conduct.

“I’m not in any position to tell anyone how my behavior made them feel,” he said in an email to The New York Times. “I have no excuses for my alcoholism or aggression, only rationalizations. I have been abusive to myself and everyone around me for years. I have a history of hurting the people closest to me. I’m ashamed of that history and am sorry to those I hurt. There is nothing else I can really say.”

The lawsuit, filed in Los Angeles Superior Court, says that Mr. LaBeouf knowingly gave Ms. Barnett a sexually transmitted disease. It accuses him of “relentless abuse,” including sexual battery, assault and infliction of emotional distress.

Mr. LaBeouf and his representative did not immediately respond to a request for comment on the lawsuit.

Karolyn Pho, a stylist who is another of Mr. LaBeouf’s former girlfriends, described similarly tumultuous experiences to The Times, some of which are also outlined in the lawsuit. Once, the suit says, he drunkenly pinned her to a bed and head-butted her, enough that she bled. Afterward, she began to grapple with the idea that he was abusing her. “So much goes into breaking down a man or woman to make them OK with a certain kind of treatment,” she said in an interview.

Presented with a detailed account of the claims that the women made against him, in interviews and subsequently in the lawsuit, Mr. LaBeouf, responding in a separate email, wrote that “many of these allegations are not true.” But, he continued, he owed the women “the opportunity to air their statements publicly and accept accountability for those things I have done.”

He added that he was “a sober member of a 12-step program” and in therapy. “I am not cured of my PTSD and alcoholism,” he wrote, “but I am committed to doing what I need to do to recover, and I will forever be sorry to the people that I may have harmed along the way.”

Mr. LaBeouf has a long history of turbulent behavior. He has been arrested several times on charges that have been dismissed, including assault and disorderly conduct, according to newspaper reports and public records. In 2015, strangers recorded video of him arguing with his girlfriend at the time, the actress Mia Goth, telling her, “This is the kind of thing that makes a person abusive.” After the men recording Mr. LaBeouf gave him a ride, he told them: “If I’d have stayed there, I would’ve killed her,” according to the video.

Ms. Barnett said Mr. LaBeouf would squeeze or grab her to the point of bruising. But she did not go to the police, she said, first out of a misguided concern about harming his career, and later because she thought her account would not be taken seriously, and it would be futile.

Though many states have laws that treat gender-based, sexual or domestic violence as a civil rights violation, tort suits of the kind Ms. Barnett is pursuing, with a daunting account of painful moments, are relatively uncommon; most often, allegations arise amid divorce or custody proceedings, or while seeking orders of protection. But there has been a slight uptick in civil claims since the #MeToo movement, amid more attention on the complex nature of abuse, said Julie Goldscheid, a law professor at CUNY Law School who studies gender violence and civil rights.