(NewsNation) — WNBA star Brittney Griner returned to the U.S. Friday after a high-profile prisoner swap with Russia. But the wrongful detention of Americans has become more widespread and harder to resolve over the past decade, according to a new study by the James W. Foley Legacy Foundation.

The study from the group, which advocates to free hostages and the wrongfully detained, found a 60% increase in the average length of a U.S. national’s captivity when taken hostage over the past 11 years. Several cases involved Americans being detained for more than 2,000 days spanning three presidential administrations.

“It is important to evaluate whether the structures currently in place are helping or hindering the recovery of Americans held hostage abroad,” the report stated.

The detention of an American is considered unlawful or “wrongful” based on criteria found in the Robert Levinson Hostage-Recovery and Hostage-Taking Accountability Act that was adopted into law in December 2020, according to the U.S. Department of State.

The Act lists 11 different cases in which an American can be considered “wrongfully detained,” including but not limited to receiving credible information indicating the detainee’s innocence, the detainee being held solely because they are American or even if an individual is being detained in violation of the laws of the detaining country.

On Thursday, Griner was freed from Russian captivity in one of two recent prison swaps. In April, Russia released former U.S. Marine Trevor Reed after more than two years of detainment overseas.

Another former U.S. Marine, Paul Whelan, continues to be detained in Russia, where he has been held for nearly four years on suspicion of spying. Whelan’s family maintains he was attending a wedding in Moscow when he was detained.

Another American school teacher, 61-year-old Marc Fogel, has been held prisoner in Russia since last year. He was sentenced to 14 years in prison after being caught with 17 grams of cannabis that was medically prescribed by his doctors.

Anne Fogel, Marc’s sister, told NewsNation’s CUOMO that while she’s happy about the release of Griner, her family has mixed emotions.

An estimated 60 Americans are being held prisoner across the world, mostly by foreign adversaries like Iran, Russia and Venezuela, the foundation reported.

Here are some of the names of Americans who have been wrongfully detained or held hostage abroad:

Austin Tice

Paul Overby

Siamak Namazi

Jeffery Woodke

Majd Kamalmaz

Dr. Walid Fitaihi

Paul Whelan

Kai Li

Aziza Al-Yousef

Salah Al-Haidar

Bader Al-Ibrahim

Paul Rusesabagina

Alina Lopez

Airan Berry

Luke Denman

Mark Swidan

Shahab Dalili

Emad Sharghi

Zack Shahin

Eyvin Hernandez

Theary Seng

Marc Fogel

John Cao

David Lin

Morad Tahbaz

Baquer Namazi

NewsNation’s Katie Smith, Liz Jassin and Alex Caprariello contributed to this report.