(CNN) A lawyer for Paul Whelan , a US citizen charged in Russia with espionage, said Thursday he has filed an appeal in court against Whelan's detention without bail, which he called unwarranted and excessive.

Whelan, 48, a Michigan resident and corporate security director, was arrested December 28 in Moscow on suspicion of carrying out an act of espionage , Russia has said. His family rejects the accusation, asserting that he was in Russia only for a vacation.

The Russian government announced the arrest Monday but has not officially released further details about the specific allegations against Whelan.

Whelan's lawyer in Russia, Vladimir Zherebenkov, said Whelan has been charged and detained in Moscow's Lefortovo prison since the day of his arrest.

"I filed a complaint with the Lefortovo court, because I believe that the detention (without bail) is excessive and unwarranted," Zherebenkov said on Thursday. "We are asking for bail."

US source: No apparent ties to intel operations

Paul Whelan does not appear to have any connection to any national intelligence operation, a diplomatic source familiar with the case told CNN on Friday.

Whelan entered Russia on his US passport and it remains unclear what exactly he might have been trying to do on his own in Russia, the source said.

When asked whether Whelan had any ties to US intelligence, the CIA and office of the Director of National Intelligence referred the inquiries to the State Department.

Whelan's twin brother, David Whelan, wrote in an op-ed in The Washington Post that his brother is not a spy. "He is many things to many people, but he is not a spy."

Russian report: He had classified information

An unidentified source told Russian news site Rosbalt that Whelan was arrested in his hotel room with a flash drive containing classified information.

Rosbalt said its report was based on a source in the Russian special services.

CNN has not independently verified that these are the official Russian allegations against Whelan.

Rosbalt reported that on the day of his arrest, Whelan met a Russian citizen, a person he had known for a long time and whom he had repeatedly tried to recruit as an agent to obtain information about the staff of Russian intelligence agencies, according to its source.

Whelan received an electronic device with a list of employees of a classified department, and then Federal Security Service officers broke into his room at the Metropol Hotel five minutes later and detained him, Rosbalt reported.

Whelan grew up in the United States after being born in Canada to parents from the United Kingdom -- and appears to have citizenship in all three countries.

After serving 14 years in the US Marine Corps Reserve, Whelan is now the director of global security for Michigan-based automotive components supplier BorgWarner

If found guilty of espionage, he faces 10 to 20 years in prison, Russia's state-run news agency TASS reported

Whelan is upbeat, attorney says

Whelan feels well and is in a good mood, and the prison has treated him well, Zherebenkov said. He didn't discuss the details of the charges but said he had appealed Whelan's detention and applied for bail.

It could be half a year before the case goes to trial, the lawyer said.

Whelan's family dismisses notions that he was a spy, saying the discharged US Marines reservist was in Russia to attend a wedding for a fellow former US service member and a Russian woman.

The United States has told Russia it expects more information about the charge, and it will demand Whelan's release "if the detention is not appropriate," US Secretary of State Mike Pompeo said Wednesday. He told Newsmax on Thursday the "most senior levels" of government are involved and "we're doing everything we can to make sure that he's treated appropriately and that we get the information we need."

US Ambassador to Russia Jon Huntsman visited Whelan in prison on Wednesday and spoke with his family by telephone after the visit, a US State Department spokesman said.

"We are relieved and very pleased to know that staff of the US Embassy in Moscow have been given consular access to Paul and confirmed that he is safe," Paul's twin brother, David Whalen, said Friday in a statement released on behalf of the family.

"We deeply appreciate Ambassador Huntsman's commitment to stay in regular contact with Paul during his detention and his assurances that Paul's rights will be respected."

The UK Foreign Office said Friday its staff also has asked Russia to let its consular officials visit Whelan after receiving a request for assistance from him.

Canada's global affairs department told CNN, in response to a question about Whelan, that it is "aware that a Canadian citizen has been arrested in Russia." Citing privacy laws, it declined to comment further.

Whelan's arrest came 15 days after alleged Russian spy Maria Butina pleaded guilty in US federal court to trying to infiltrate political circles and influence US relations with Russia. Former US national security officials have speculated to CNN that the arrest of Whelan, spy or not, could be Russian government retaliation for Butina's prosecution.

Family says he was with wedding party shortly before arrest

Whelan's online footprint and information from his family paint a picture of a former military man and corporate security expert who had some familiarity with Russia, traveling and collecting friends there, and having an account on a Russian social media platform similar to Facebook.

JUST WATCHED Brother of US citizen detained in Russia speaks with CNN Replay More Videos ... MUST WATCH Brother of US citizen detained in Russia speaks with CNN 04:43

Whelan was in Russia to attend a retired fellow Marine's wedding and serve as a guide to wedding guests who had not visited Russia previously, according to David Whelan.

Paul Whelan flew to Moscow on December 22. By Friday, he was with the wedding party at the Kremlin, acting as a tour guide, his brother said.

When Paul didn't arrive at the wedding later Friday, the newlyweds filed a missing-person report with Russian authorities, the brother said.

Before his arrest, Paul had plans to check into a St. Petersburg hotel on Wednesday and fly home Sunday, David said.

Who is Paul Whelan?

Whelan lives in Novi, Michigan. As a Marine reservist, he served two tours in Iraq, in 2004 and 2006.

The onetime staff sergeant's service with the Reserve ended in 2008 with a bad-conduct discharge for wrongfully using another person's Social Security number and writing bad checks, according to military court documents. A military judge found him guilty of attempted larceny and three specifications of dereliction of duty.

He's had a career in corporate security, his brother said, including at BorgWarner since 2017.

Whelan's job entails making sure BorgWarner facilities are physically secure -- that they can't be stolen from or broken into -- his brother says. The company confirmed his employment, but noted BorgWarner has no facilities in Russia.

Friends and trips in Russia

Few details about Whelan's previous travels to Russia were immediately available. But he's no stranger to the country, having been there several times for work and personal business, his brother said.

During his 2006 Iraq tour with the 3rd Marine Aircraft Wing, he used his two weeks' leave to visit Russia.

2007 online Marine Corps article says he spent the leave "experiencing the post-Soviet era of Moscow and St. Petersburg." An attached photo shows him standing across the Moscow River from the Kremlin.

Paul Whelan spent two weeks' military leave in Russia in 2006, according to an article posted on the US Marine Corps website. This photo accompanying the article shows Whelan in front of the Kremlin during that trip.

It also appears that Whelan had been using a Russian social media platform for the past 13 years.

Photos showing Whelan have been uploaded to the VKontakte, or VK, social network from around the world for years, in a profile under his name.

The last status update on the page reads "next stop, Moscow..."

When asked about the VKontakte page, Whelan's brother said he couldn't confirm the account, but accepts that his brother likely had one, because he made friends in the country.

"I knew over the years he had met Russians," David Whelan said on Wednesday. "I don't know whether it was through work or through his Marine activity. But he'd met people while he'd been traveling around, and he connected with them on social media. And he would tell us that when he was traveling he would go visit those people."

Yet the brother said he didn't know "that Russia was specifically a place he'd like to go."

"He liked to travel wherever he could. He has been to India and Iceland, all over the place. He has friends in Russia, so that would be an extra draw, people he's met on social media, but I don't know that Russia was a particular place of return for him," David Whelan said.

Correction: This story has been updated to clarify that Whelan's lawyer is appealing his detention without bail, not his arrest. It also updates the attorney's quote regarding this.