With increasing concerns about the coronavirus spreading in Iran’s overcrowded prisons, Michael White, an American citizen detained in Iran since 2018, has been temporarily released, Secretary of State Mike Pompeo said Thursday, although four other American imprisoned in Iran apparently remain in custody.

White must remain in Iran and is under the custody of the Swiss embassy, where he will undergo a series of medical checks. He has cancer that is likely to have gone untreated because of Iran’s prison conditions, making him especially vulnerable to the coronavirus outbreak.

“We are thrilled to learn that Michael White has been released on medical furlough,” said Margaux Ewen, executive director at the James W. Foley Legacy Foundation, which advocates for the safe return of all Americans unjustly detained abroad. “[But] we remain concerned for his health and well-being.”

Iran has temporarily released at least 70,000 detainees, ordering them to self-contain at home in response to the coronavirus, yet American citizens detained in Iran remain imprisoned and exposed to the outbreak, according to medical and human rights experts.

“Reports that COVID-19 has spread to Iranian prisons are deeply troubling and demand nothing less than the full and immediate release of all American citizens,” Secretary of State Mike Pompeo said last week. “Their detention amid increasingly deteriorating conditions defies basic human decency.”

There are over 17,000 confirmed cases of coronavirus in Iran, according to Worldometer, a platform offering live statistics from around the world. Since Tuesday, the country recorded 988 deaths, according to Kianush Jahanpur, a spokesman for the Iranian Health Ministry.

In an effort to limit the outbreak in overcrowded prisons, Iran ordered the temporary release of prisoners with sentences of less than five years, according to Kamiar Alaei, co-president of the Institute for International Health and Education and an epidemiologist previously based in Iran. Political prisoners were excluded from the temporary release.

“Prison populations are extremely vulnerable to contagious diseases due to their confined living spaces and should be protected along with all other vulnerable groups and communities during health crises,” said Jasmin Ramsey, director of communications at the Center for Human Rights in Iran.

Pompeo said foreign powers should not provide Iran with any humanitarian aid until they release all foreign detainees in response to the coronavirus outbreak. He also said the United States will hold the Iranian regime accountable for the wrongful imprisonment of American citizens.

Besides White, there are at least another four Americans imprisoned in Iran.

Baquer Namazi was convicted along with his son, Siamak, in October 2016 for collaborating with a hostile government, the United States. Robert Levinson is a former FBI agent detained in Iran’s Kish Island in 2007 and considered to be the longest-held American hostage in history, but little is known of his current situation. Morad Tahbaz, an Iranian-American, was arrested in January 2018 and sentenced to 10 years in prison for allegedly conducting espionage.

“I am begging as a human being to please allow my brother as well as others who have been exposed to the virus to be given a [temporary release to self-contain,]” said Babak Namazi, the son of Baquer Namazi and brother of Siamak Namazi. “We’re desperate. I cannot underscore enough the danger that they’re exposed to.”

Iranian prisons lack testing kits, medicine and disinfectants. Prisons are also overcrowded, reaching at least twice their capacity, said Alaei, who was imprisoned in Iran between 2008 and 2010.

“Definitely some of the people who visit prisoners and the prison staff can easily bring those viruses [into the facilities] and there is definitely a huge outbreak among prisoners because of the limited health care system,” said Alaei.

Ramsey urged that all detainees have unrestricted access to adequate medical treatment as new coronavirus cases are continuously detected in prisons.

“We hope that Americans Siamak and Baquer Namazi and Morad Tahbaz can be granted at the very least the same medical furlough [as Michael White] to protect them from the virus and receive any medical attention they might need,” said Ewen. “But we are also particularly concerned for the well-being of missing American Bob Levinson, the longest-held hostage in US history.”

“These people have a basic right to protect themselves,” said Alaei. “This is the responsibility of the prison organization to either provide [political prisoners] a temporary release or to make sure to protect their lives and access to healthcare services.”