It has been one week since the family of Paul Whelan, an American detained in Russia since 2018, heard from the former US marine.
Mr Whelan calls his parents daily and his family said he would never miss calling on Thanksgiving.
“It's very weird that he wouldn't call on Thanksgiving,” said his twin brother, David. “Obviously it's an important holiday for our family and he's very sentimental about those sorts of holidays.”
Mr Whelan’s father turned 85 on Wednesday, a milestone his family said he would never miss the opportunity to call for, if he was able.
David Whelan said officials had told the US embassy that his brother had been moved to the prison hospital, but he said that does not explain the lack of communication.
“On at least one other hospital visit, he has been able to call home,” he told The National.
Though Mr Whelan has gone long stretches without communicating with his family before, it is rare that they go unexplained.
“It's really disconcerting that suddenly he would disappear,” his brother said.
US officials said they are trying to confirm where Mr Whelan is being held but they have not yet been successful.
“We have been trying to get more information about Mr Whelan's condition and his whereabouts,” National Security Council spokesman John Kirby on Wednesday.
“And as we speak this morning, regrettably, we do not have an update specifically about where he is or what condition he's in.”
Mr Whelan is serving a 16-year sentence for espionage after being detained four years ago while visiting Moscow for a friend’s wedding.
“We certainly share the anxiety and the concern of the family,” Mr Kirby said.
“We are deeply concerned about the lack of information and the lack of contact from Paul, and we're working on it as hard as we can through the diplomatic channels.”
The families of Mr Whelan and of basketball star Brittney Griner, who is serving a nine-year sentence in a Russian prison after being convicted of drug-trafficking charges, have called on US President Joe Biden to find a way to bring them home.
In April, the Biden administration successfully negotiated the release of Trevor Reed, another former marine held in Russia, in exchange for Konstantin Yaroshenko, a Russian pilot who was serving a 20-year jail sentence in the US for conspiring to import cocaine.
Reuters contributed to this report