(CNN) As American security executive Paul Whelan enters his third month of detention in a Moscow prison with no end in sight, sources with knowledge of the situation paint a grim picture of Russia's interference in US efforts to assist him.

Whelan, an American who Russia says was arrested for espionage, has been imprisoned in Lefortovo detention center since late December 2018. While the US -- along with Canada, Ireland and the United Kingdom, as Whelan is also a citizen of those nations -- has been granted some consular access, two sources tell CNN his visits with American Embassy officials have been strictly limited by the Russian government.

The sources say that, in an unusual move, US Embassy staff are not allowed to give him information that includes proper names or anything on paper.

"They're not allowed to say names out loud," one source tells CNN.

Whelan is not allowed any visits from friends or family, the sources say, and he hasn't been given any of the more than 100 letters his family members have sent him. He has been able to send letters home, but the assumption is that they are being read by the Russian authorities.

Also, Whelan is not allowed to discuss why he's there or details of his case. The sources say US officials have complied with the rules so they don't face further access restrictions.

'Exceptional' level of obstruction

Ryan Fayhee, a former Department of Justice lawyer working pro bono on the family's behalf, says the level of obstruction is "exceptional."

"This is not the way it usually works internationally, and not the way it should work. We don't do that to them," he tells CNN.

Perhaps most disturbingly, Fayhee says, Whelan has, to the best of his knowledge, not been officially charged with any crime. His case has not yet been handed over to the prosecutor's office, although Russia said it formally charged Whelan in early January.

"The system works differently from the American system, and based on the best information we have, they have not yet referred him over to the prosecutor's office. They're investigating him for espionage. The FSB investigator is running the process," he says. "There have been no formal or public charges. But there's no real way of knowing. It's difficult to know exactly what's going on. If there is some evidence of a crime, we don't know it."

Another source working with the family echoed this uncertainty.

"The Russians say that they have charged him. They have not done so publicly, so we don't know if that's true or not," they say.

According to Fayhee and sources with knowledge of his case, Whelan's Russian attorney is not one of his choosing and has been appointed by the Russian security service, the FSB. Two of the sources say the lawyer, Vladimir Zherebenkov, speaks no English --a translator is provided during his meetings with Whelan. One tells CNN that Whelan's next meeting with Zherebenkov has not been scheduled and isn't expected to take place until at least mid-March.

Whelan has been unable to sign a Privacy Act Waiver, meaning the State Department is unable to share details about his case. Following the US' last consular visit with Whelan, the spokesperson for the US Embassy in Moscow expressed concern at Russia's obstruction of the waiver.

"We are strongly concerned about the delay in allowing Mr. Whelan to provide a signed Privacy Act Waiver (PAW). U.S. privacy laws require a consular official obtain permission from someone before we can release any information about their case, and this is done routinely," Andrea Kalan wrote on Twitter on February 22, a day after the visit. "In every other instance, we have been able to obtain a signed PAW, but in Mr. Whelan's case, the Investigative Committee is not allowing this to happen. Why is this case any different? Consular access without being able to do true consular support is not real access."

A State Department spokesperson said at the time that the US had "expressed our concerns through diplomatic channels."

On February 22, a Russian court extended Whelan's detention by three months, until at least late May, Russian state news agency TASS reported. Russia accused the Michigan resident and former Marine of carrying out an act of espionage. His lawyer told CNN in late January that Whelan was found with "evidence that constitutes state secrets." Before his arrest, Whelan had been given a thumb drive which he believed contained vacation photographs, Zherebenkov said. Whelan did not inspect the flash drive before his detention and was not aware it contained classified information, the lawyer said. Sources tell CNN there has been no independent confirmation that Whelan received such a thumb drive.

Being held as collateral?

Fayhee and the other sources tell CNN they believe Whelan is being held as collateral in retaliation for the US arrest of accused Russian spy Maria Butina.

Russia has made it clear it wants Butina released and has accused the US of torturing her while in detention. The Russian government-aligned media has painted a saintly picture of Butina, claiming she has become very religious while in jail. They say she reads several Christian books a week, has been meeting with an Orthodox priest and wants to become a teacher when she returns to Russia.

Whelan's team points out that unlike the detained American, Butina has unfettered access to communications and a lawyer of her own choosing. Meanwhile, Whelan is being isolated through Russia's obstruction, Fayhee says.

"The goal is very clear -- they're trying to extract some benefit from the US. The problem is, Paul Whelan is no spy," Fayhee says.

Whelan's family also denies that he is a spy. Multiple sources with knowledge of the situation have echoed this claim. His family says he was in Moscow for a vacation, including the wedding of a friend. Two sources confirm that this wedding did take place and the groom was the one who reported Whelan missing.

During his detention, Whelan has not been kept in solitary confinement, but sources say his cellmate speaks no English. He is allowed to send letters, in which he tells jokes and makes references his family would recognize, so they know it's really him, the sources say. However, he is afraid to tell them anything about his case or his treatment there, for fear it will make his situation worse.

Whelan has not been granted visits by family or friends nor is he allowed to receive letters from them.

"He has only received notes that we have sent by way of his lawyers, or verbal communications from consular visits," one of the sources says.

Tuesday was Whelan's 49th birthday, his brother David said. He didn't receive the birthday cards they sent him.

CORRECTION: This story has been updated to correct the spelling of Ryan Fayhee's last name.