It was May 31, 2005, at 4 a.m., when Beth Holloway first confronted Joran van der Sloot, a Dutch student who was one of three people who had been seen, just hours earlier, leaving a nightclub in Aruba with her daughter, Natalee Holloway.

Standing outside the Holiday Inn where Natalee, 18, had been staying with other recent graduates of her Alabama high school on a class trip, Mr. van der Sloot repeatedly slammed his hands against his chest, saying, “What do you want me to do?”

Ms. Holloway recalled holding her daughter’s senior class portrait and telling him, “I want my daughter back.”

“He had such power over me,” Ms. Holloway, 63, said in an interview on Friday, “and I knew he had the answers, and I knew that he was well aware of what had happened to her.”

Ms. Holloway spent 18 years trying to find out what happened to Natalee, in a quest that took her to Holland and back to Aruba and put her in touch with senior government officials like Secretary of State Condoleezza Rice and celebrities like Dog the Bounty Hunter. Along the way, her daughter’s case became a staple of cable news coverage and the subject of two Lifetime movies, multiple books and fictionalized accounts.

But the answer did not come until Wednesday, when Mr. van der Sloot, now 36, finally admitted that he brutally killed Natalee Holloway on a beach in Aruba after she rejected his sexual advances.

He gave his account in a statement that was filed in federal court in Birmingham, Ala., as he pleaded guilty to extortion and wire fraud charges. Prosecutors said he demanded $250,000 from Beth Holloway in 2010 in exchange for information about the location of her daughter’s remains. He received only $25,100 from her after providing false information, according to prosecutors.

Despite an extensive search, Ms. Holloway’s remains have never been found. She was declared legally dead in 2012. No one has ever been charged in her killing.

But Beth Holloway said that Mr. van der Sloot’s admission that he killed her daughter and dragged her body into the water means that “my never-ending nightmare is over.”

“His power is gone,” she said. “His hold over me is gone. You know, it took 18 years for us to shift roles to where now I feel like I was the victor over him.”

Mr. van der Sloot was sentenced to 20 years in the extortion case. He will serve the sentence concurrently with a 28-year prison sentence he is serving in Peru for the 2010 murder of a 21-year-old student, Stephany Flores.

Beth Holloway said Mr. van der Sloot’s offer to help her in exchange for $250,000 had been communicated to her through the television host Greta Van Susteren.

Ms. Holloway said she agreed because she would listen to anyone who had a tip, a lead or a theory, even though they had led her down countless “rabbit holes.”

“I never said no,” she said, adding, “I just didn’t want to have any regrets.”

In his statement, Mr. van Der Sloot described wanting to be dropped off with Natalee Holloway a distance from the hotel where she was staying with more than 100 recent graduates of Mountain Brook High School in Alabama so he “might still get a chance to, to be with her.” He said they began kissing while lying on the beach, but she refused further sexual advances.

When he persisted, he said, she kneed him in the crotch, and he kicked her in the face. He then smashed her head with a large cinder block, pushed her body into the water and went home, he said.

In the interview on Friday, Beth Holloway began to cry as she talked about the graphic account of her daughter’s death. But she said she also found comfort in “Natalee’s fight to stand her ground” against Mr. van der Sloot’s advances.

“She stood up for what is right, and what she believed in,” she said. “She was willing to die for it. I mean, she lost her life, but she fought her perpetrator. So I’m really proud of her.”

Ms. Holloway said that Mr. van der Sloot apologized in court on Wednesday, but “it was a hollow apology for himself.”

His account of her daughter’s death was more important, she said, crediting the work of F.B.I. agents, federal prosecutors and the U.S. Marshals Service.

“I had no justice and no answers for 18 years,” she said. “And so now, 18 years later, I can say today, I do have answers that I had been so desperately searching for all these years. And with those answers, we finally got justice for Natalee.”

Over the years, Ms. Holloway, a speech pathologist who lives in Homewood, Ala., has spoken frequently about missing-persons cases and travel safety at churches, high schools and law enforcement conferences. She also published a book about her experience in 2009, “Loving Natalee: The True Story of the Aruba Kidnapping and Its Aftermath.” In 2011, she hosted a true-crime television series, “Vanished with Beth Holloway.”

The extensive attention given to her daughter’s case has been criticized as an example of the news media’s selective focus on cases of missing white women, compared to coverage of similar cases of women from other ethnic groups and backgrounds.

Ms. Holloway said she hoped her daughter’s case would offer some encouragement to “other families that are hurting and are seeking answers for their loved ones.”

She said that she tries to help families that come directly to her for help. “If the family or family member reaches out to me, I go to work,” she said.