Dutch national Paul van der Sloot, 52, a judge in training, became the fifth person detained, and joins 17-year-old Joran van der Sloot and three other young men already in custody.
No one has been charged in the case.
The elder van der Sloot "is a suspect in the disappearance" of 18-year-old Natalee Holloway, said Mariaine Croes, spokeswoman for the Attorney General's office on the Dutch Caribbean island.
Joran's mother, meanwhile, told The Associated Press on Thursday that her son had changed his story, admitting to her that he was alone with Holloway on a beach the night she vanished — and did not drop her off at her hotel as he had told police. But Joran insisted that he did not hurt her, Anita van der Sloot said.
Paul van der Sloot was arrested shortly after he and his wife left the San Nicolas prison where Joran is being held, his wife told the AP. She said they received a call from neighbors saying police were waiting for them at their home in Noord, northwest of the capital, Oranjestad.
She said she called Police Superintendent Jan van der Straaten, who asked the couple to come to the police station. When they arrived, authorities "took my husband into custody as a suspect," Anita van der Sloot, 49, said, adding, "I don't know what to think."
Holloway's mother, Beth Holloway Twitty, was happy with the development.
"We are very pleased that the investigation is progressing," she said. "We feel like this will lead to more information to give us the answers we need for finding Natalee."
Joran, 17, and friends Deepak Kalpoe, 21, and Satish Kalpoe, 18, told police initially that they dropped off Natalee Holloway at the Holiday Inn where she was staying.
His mother insisted her husband had done nothing wrong and said Aruban authorities had arrested him because they were under pressure from the U.S. government and the news media to produce results.
"My husband is a man of integrity who has been working in the justice system 15 years and was taken without evidence," Anita van der Sloot said. "How can this happen? This is not about Natalee anymore. It's about enormous pressure from the [United] States and the media."
Authorities did not immediately respond to van der Sloot's allegation.
And CBS' Aleen Sirgany reports Anita van der Sloot said, of her husband, "He did everything they asked, he's the most honest person I know. I love them both."
Meanwhile, Holloway's family personally continued the search for the missing Alabama high school student.
"It's funny that we talk about how small this island is and yet so large when you are looking for one individual," Holloway's aunt Linda Allison told CBS News Correspondent Kelly Cobiella.
Repeated searches of the island have produced no trace of Holloway, who went missing in the early hours of May 30, the last day of a high school graduation trip with 124 other students. Her passport and packed suitcase were found in her hotel room.
Joran van der Sloot has been in custody since June 9, along with Deepak Kalpoe and Satish Kalpoe, Surinamese brothers. Authorities also have arrested a 26-year-old party boat disc jockey, Steve Gregory Croes.
Police say Joran met Holloway at a casino two days before her disappearance. The three young men told authorities that they took her from a popular restaurant to a beach, where Joran and Holloway were kissing in the back of the car, then dropped Natalee at the Holiday Inn about 2 a.m.