(NewsNation) – The imprisonment of a Wall Street Journal reporter Evan Gershkovichwas arrested on espionage charges extended until January 30thsaid Russian news agencies.

Danielle Gershkovich, Evan’s sister, said the family exchanged letters with her brother about once a week.

“It’s just a lifeline – supporting him and supporting us,” she said.

Danielle Gershkovich emphasized that she and her family try to keep Evan updated on family news, but there is also “a lot of sibling humor.”

“We talked about TV shows, we looked at books he reads,” she said. “I can tell from the letters that he works very, very hard to stay in good spirits and I’m just so proud of him for that.”

She added that it was “bittersweet” to see her brother on camera behind bars and that she’s always making sure “he’s holding up well.”

“I think that’s our first concern – does he look healthy? Is he still in a good mood? It’s always a bittersweet moment to see him in front of the camera and see him behind bars or in that glass aquarium as they call it that he’ll be standing in, but also being able to see him, so the feelings are very mixed.” says Danielle said Gerschkowitsch.

Authorities said details of the criminal case against Evan Gershkovich are secret.

Paul Beckett, deputy editor of the Wall Street Journal, stressed that Russian authorities could still extend his pretrial detention for up to a year or longer, but they hoped U.S. authorities would bring him home soon.

“The system is, as you can imagine, quite flexible. So we hope the US can find a way to convince the Russians to return an innocent man to his family and to the newsroom. Long before it gets to that point. “250 days is 250 too many and it’s time for him to come home,” he said.

Beckett added: “I think he’s working really hard to make sure he’s OK. And his extraordinary family provides him with tremendous support and inspires us all to do everything we can to bring him home.”

Gerschkowitsch, 32, was arrested in March during a reporting trip to the Russian city of Yekaterinburg, about 1,200 miles east of Moscow. The Russian Federal Security Service claimed that the reporter “collected information on behalf of the American side that constitutes a state secret about the activities of one of the enterprises of the Russian military-industrial complex.”

Gershkovich and the Journal deny the allegations and the U.S. government has declared him unlawfully detained. Russian authorities have not provided any concrete evidence to support the espionage allegations.

Gershkovich is the first American reporter to be charged with espionage in Russia since Nicholas Daniloff, a Moscow correspondent for US News and World Report, was arrested by the KGB in 1986. He is being held in Moscow’s Lefortovo prison, which is notorious for its harsh conditions.

The Russian Foreign Ministry has said it will not consider swapping Gershkovich until after a verdict in his trial. In Russia, espionage trials can last more than a year.

The Associated Press contributed to this report.