Former U.S. Marine Paul Whelan, who was detained on suspicion of spying, stands inside a defendants' cage as he attends a court hearing regarding the extension of his detention, in Moscow, Russia, May 24, 2019. REUTERS

Moscow -- A Russian court has extended the detention of a former U.S. Marine arrested on espionage charges. The court ruled on Friday to keep Michigan resident Paul Whelan, who also holds British, Irish and Canadian citizenship, behind bars for three more months.

Whelan told reporters in court that he has been threatened and subjected to "abuses and harassment" in prison.

"I want to tell the world that I am a victim of political kidnap and ransom," Whelan said to gathered television cameras from behind bars in the courtroom. "This is retaliation for sanctions. There is absolutely no legitimacy. I have been threatened. There are abuses and harassment that I am constantly subject to. There is a case for isolation. I have not had a shower in two weeks, I can't use a barber I have to cut my own hair. I can't have medical treatment, I can't have dental treatment… This is typical POW Chapter 1 isolation technique. They are trying to run me down so that I will talk to them. This is Salem witch trial mentality and I am asking the world press to go to London, to go to Washington, to go to Dublin."

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The American was arrested in a hotel room in the Russian capital at the end of December and charged with espionage, which carries up to 20 years in prison in Russia. The court's decision on Friday was the second time it had granted a three-month extension of Whelan's arrest.

Whelan's lawyer has said his client was handed a flash drive that had on it classified information he didn't know about.

U.S. intelligence and State Department sources have told CBS News they're confident Whelan isn't a spy.

Whelan's family, who also deny the accusations against him, have said his Russian lawyer was chosen for him.

That lawyer, Vladimir Zherebenkov, told CBS News' Elizabeth Palmer outside the court in Feburary, when Whelan was arraigned, that his client was found at his hotel at the time of his arrest with a flash drive containing Russian "state secrets."

He didn't, however, explain how Whelan came to be in possession of the information on the drive and said his client wasn't aware that the Russian government considered the material sensitive.

"I've worked as an investigator for 20 years and if I do have a smoking gun, I will then put my strongest card on the table, to crush the opponent. In this case I don't see any smoking gun," Zherebenkov said after the February hearing.

He said the prosecution didn't "have strong evidence to back up the charges."

Whelan worked for a Michigan-based auto parts company since 2017, becoming its global security director. He spent 14 years in the U.S. Marine Corps before being discharged in 2008 for bad conduct, according to the military.