A former U.S. Marine detained in Russia is at risk of dying in custody, according to his family and an advocacy group, which said he had been mistreated and was in serious condition in a civilian hospital.

Robert Gilman, 32, has been detained since 2022, when a Russian court in Voronezh convicted him of assaulting a police officer and sentenced him to four and a half years in a penal colony. The sentence was extended to 10 years last year, after the authorities said that Mr. Gilman had assaulted prison staff and an investigator, according to Russian news agencies.

Mr. Gilman’s family and Global Reach, a nonprofit advocacy group representing them, have accused the Russian authorities of wrongfully detaining him for political purposes because he is American.

Since June, Mr. Gilman had been in a “dissociative stupor,” in which a patient is unresponsive without an apparent physical cause, said Eric Lebson, chief strategy officer for Global Reach, who has been in contact with Mr. Gilman’s family. Mr. Gilman’s condition was first reported by Reuters on Thursday.

Mr. Gilman was being fed through a tube, was running a high fever and had hypertension, Mr. Lebson said in an interview on Friday.

“Gilman is in a life-or-death situation right now,” he said.

The detainee’s mother, Nina Gilman, flew to Russia from her home in Massachusetts to see him, but has been denied entry by hospital officials.

“That’s very concerning to us,” Mr. Lebson said.

Mr. Gilman’s family has said that he has been force-fed drugs, deprived of food and sleep, and goaded into conflicts with guards while in custody to add more charges, according to Mr. Lebson. A Russian news agency reported in July that Mr. Gilman had been moved to a hospital because of his “deteriorating health.”

A lawyer for Mr. Gilman could not be reached for comment on Friday, and a spokeswoman for Russia’s Foreign Ministry did not respond to a request for comment.

The State Department said in a statement that it was “deeply concerned about Robert Gilman’s health and continued detention in Russia.” The government had raised his case repeatedly with the Russian government, the statement said, and requested Mr. Gilman’s release on humanitarian grounds.

“I ​am more afraid for my brother than I have been since this horrible nightmare started,” Lexie Hudson, Mr. Gilman’s sister, said in a statement provided by Global Reach. “I’m worried that I ⁠may never see him again if the Russians don’t release him. I’m worried that he could die.”

She said that the last time she had spoken to her brother was before his arrest. “We sometimes can send him letters and get replies, but I want him to hear my voice so he knows in his heart that he is not forgotten and we are fighting for him ever single day,” she added.

Mr. Gilman’s parents moved from Russia to Dracut, Mass., his father, Vladimir Gilman, wrote in a 2024 Boston Globe article. Robert Gilman was born in Poland and raised in Massachusetts, and served in the U.S. Marines. He had been traveling to Moldova in early 2022 to explore a teaching opportunity when he fell ill while transiting through Russia, his father wrote.

Mr. Gilman decided to recuperate in Russia and visit an aunt who lived there, but while on a train, his father wrote, he fell ill and was taken to a local police station, where he accidentally kicked a police officer. “This event would normally result in a fine or a suspended sentence, if anything,” Mr. Gilman’s father wrote in the 2024 article. “But Robert was singled out because he is an American.”

The Foley Foundation, a nonprofit that advocates on behalf of American hostages abroad, said that Mr. Gilman was one of at least seven American citizens it considered wrongfully detained in Russia. Only one person from that group, Stephen Hubbard, has been designated as “wrongfully detained” by the State Department.

Mr. Gilman’s family asked the State Department in 2023 to designate him as a wrongful detainee, which would allow the U.S. government to negotiate for Mr. Gilman’s release, his sister said. It was not granted at the time, and no detainee has received that designation during this Trump administration.

At an April hearing in Voronezh, Mr. Gilman told a judge that he wanted to fight for Russia in the war in Ukraine, according to a Russian news agency. But Senators Edward Markey and Elizabeth Warren of Massachusetts, in a letter to the department shortly after, said that Russian officials had told Mr. Gilman that his government had forgotten about him and that the only way to earn his freedom was to serve in the Russian Army in Ukraine.

“He needs to come home and needs proper medical care,” said Elizabeth Richards, the director of hostage advocacy and research at the Foley Foundation. “It is not a good outcome for anyone if Robert dies. If he doesn’t come soon, that is our biggest concern.”

In recent years, several American citizens who have been designated by the State Department as wrongful detainees in Russia have been released after negotiations between Washington and Moscow. The most significant was a painstakingly negotiated prisoner swap in Ankara, Turkey, in 2024, in which 15 people imprisoned by Russia — including the Wall Street Journal reporter Evan Gershkovich — and one in Belarus were freed in return for eight people held in Western countries.

On Thursday, Senator Markey said that he feared Mr. Gilman’s condition could become permanent, or that he could die in Russian custody. He urged the U.S. and Russian governments to find a solution.

Paul Sonne and Remy Tumin contributed reporting.