Complicating matters further is the fact that Whelan holds four passports: American, Canadian, British and Irish - all of which have offered him consular support.
Russian intelligence services have publicly stated that in the event of a guilty verdict, Whelan faces between 10 and 20 years in prison.
The foreign secretary's remarks support the premise of speculation in the U.S. that Mr Whelan's arrest is a strategic move by Moscow.
Hunt adds that "we are extremely anxious about him and his family".
David, who lives in Toronto, said only US embassy officials in Russian Federation have been able to communicate with his brother.
Mr Whelan continued to work for Kelly Services, being promoted to senior manager of global security and investigations in 2010.
There has been speculation he could be used to barter for the release of a Russian citizen jailed in the U.S. after admitting acting as a secret agent for the Kremlin.
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Russian President Vladimir Putin refuted British claims the Kremlin was behind the nerve agent poisonings in Salisbury previous year.
The ministry said the United States detained Russian citizen Dmitry Makarenko on December 29 on the Northern Mariana Islands, a USA commonwealth in the Pacific Ocean, and had transferred him to Florida.
"I know him as a friendly, polite, educated, and easygoing guy", said one of his contacts, who, like the other Russians interviewed for this story, spoke on condition of anonymity because of Whelan's legal troubles. He now lives in MI and is director of global security for automotive components supplier BorgWarner.
Secretary of State Mike Pompeo said on Wednesday that Washington was trying to learn more about the situation. Both European countries are now requesting access to him, raising questions as to whether Whelan's many passports could affect potential negotiations for his release.
"Individuals should not be used as pawns of diplomatic leverage", British foreign minister Jeremy Hunt said.
"Our staff have requested consular access to a British man detained in Russian Federation after receiving a request for assistance from him".
According to his brother, Mr Whelan was in Moscow to help plan a fellow former Marine's wedding because he had visited Russian Federation several times previously. Last December, Mira Terada was detained by Finnish police on an Interpol warrant initiated by the United States, according to the Russian Embassy in Finland.
"There are no details about the circumstances of his arrest, so it's hard not to connect it as merely tugging between the two countries for whatever reason", David Whelan said in Newmarket, Ontario, north of Toronto.
State news agency Tass and private agency Interfax reported Thursday, citing unnamed sources, that Whelan has been indicted on the charges.