Cydney Mizell, an aid worker teaching English in southern Afghanistan, vanished in 2008, abducted after being driven off the side of a road and presumed dead for 15 years.

Members of her family, left with few other details of the circumstances surrounding her disappearance, wondered whether they would ever learn her fate.

Jan Mizell, her younger sister, said she would tell people: “Somebody over there knows what happened to my sister. They’re just not talking.”

But about a year ago, Ms. Mizell, 64, who lives south of Seattle, received news from the F.B.I.: Agents had collected small bone fragments belonging to Cydney in Afghanistan and would try to bring back all of her remains

The recovery of Cydney Mizell brings to an end a terrorism case that had long stymied investigators, becoming one of the oldest kidnappings that the F.B.I. has worked on in Afghanistan. It also demonstrates the intricacies of tracking down hostages, particularly in a country where the United States no longer has a presence and underscores the difficulty of finding the bodies of those lost abroad.

The F.B.I. did not make the discovery public at the time but confirmed in a statement on Saturday that Ms. Mizell’s remains were “recovered and repatriated to her family.” The effort included F.B.I. agents in the District of Columbia, as well as officials across the intelligence community who are part of the Hostage Recovery Fusion Cell, which focuses on hostage cases.

So far, no one has been charged in Ms. Mizell’s kidnapping and killing. But a former U.S. official familiar with the case said the Taliban were most likely behind the abduction and had hoped to trade her for one of their members held at the U.S. military prison at Guantánamo Bay, Cuba.

In Afghanistan, Ms. Mizell worked for the Asian Rural Life Development Foundation, teaching English at Kandahar University as well as embroidery and sewing at a girls’ school, according to a 2008 statement. She loved music, including singing and playing the piano and the guitar.

Jan Mizell said her father learned of his daughter’s disappearance in late January 2008. A shopkeeper, she recalled, had witnessed the kidnapping, relaying how Cydney and her driver had been forced off the road and taken hostage by a group of gunmen.

The kidnappers, using Ms. Mizell’s cellphone, repeatedly called the aid agency over several days. Only shortly after did the kidnappers indicate that Cydney had been killed, Jan Mizell said, though they offered few other details.

Ms. Mizell’s father died in the months after his daughter was kidnapped.

Over the years, Jan Mizell intermittently heard from the F.B.I. about the case. She received a letter from the Obama administration alerting her to changes it had made in hostage recovery efforts after families complained of haphazard communication and conflicting information from the administration. Under President Biden, the administration invited her to two video conference calls with Jake Sullivan, the national security adviser, and Secretary of State Antony J. Blinken. Ms. Mizell said the calls were for victims of terrorism and their families to ask questions about how the government handles these types of investigations.

Ms. Mizell said the F.B.I. received various tips, though nothing panned out. After receiving information about the possible whereabouts of her sister’s remains, the F.B.I. made a major push to solve the case. In 2021, the government posted a reward of up to $5 million for information about Cydney Mizell, including her “location, recovery and return,” and publicized the notice in several languages.

“I was shocked and in awe that this effort was being made,” Ms. Mizell said of the bid to find her sister.

Ms. Mizell said the reward appeared to lead to a breakthrough, with somebody stepping forward with the bone fragments. DNA taken by F.B.I. agents in 2008 from Ms. Mizell and her father confirmed it was Cydney.

Then the government took steps to locate and bring home her entire skeletal remains, including by having the bones brought through a third country. Ms. Mizell said two F.B.I. agents in April 2023 escorted the remains back to the United States draped in an American flag.

A copy of the autopsy report the F.B.I. gave her showed that her sister had been shot in the head and her skull crushed. Agents also presented her with an urn of ashes and an American flag. The agents also returned the personal journals Cydney kept during her time in Afghanistan.

“Without the agents, we would still be in some big black hole of nothingness,” Ms. Mizell said.

In October, Ms. Mizell’s family held a memorial at a Baptist church in Tacoma, Wash., where her father was once the pastor. Dozens attended the service, including F.B.I. agents. The American flag Ms. Mizell had received was on display.

Her family is expecting to finally receive an official death certificate.

Ms. Mizell said her sister, who would have turned 66 next month, sought to improve the lives of those around her.

“She was devoted to loving and helping people around the world, especially supporting women and girls in desperate situations,” she added.

Other kidnapping cases have continued to frustrate the F.B.I. In Afghanistan, investigators are still trying to track down Paul Edwin Overby Jr., an author who officials say was last seen in May 2014 in Khost city while researching a book. He had hoped to interview the leader of a militant network when he went missing. And Ryan Corbett, of Western New York, was detained by the Taliban in 2022 after visiting northern Afghanistan on a business trip.

Kitty Bennett contributed research.