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A police officer working for the Washington transport system has been charged with aiding militants of so-called Islamic State (IS).

Federal officials said Nicholas Young, 36, sent an FBI informant about $250 worth of credits for mobile messaging accounts used by IS fighters.

Mr Young's arrest marks the first time a US police officer has been charged with helping IS militants.

Authorities said there was no threat to the Washington transport system.

Mr Young had been under investigation since 2010, the FBI said.

The FBI said that Mr Young had travelled in 2011 to Libya to fight with rebels trying to overthrow President Muammar Gaddafi. He packed body armour and other military gear, authorities said.

Authorities said Mr Young was an acquaintance of Amine El Khalifi, who later pleaded guilty to plotting a suicide bombing at the US Capitol Building in 2012.

Mr Young, who has worked for the Washington transport system since 2003, was fired after he was arrested.

"Obviously, the allegations in this case are profoundly disturbing. They're disturbing to me, and they're disturbing to everyone who wears the uniform," said Paul Wiedefeld, the general manager of the transport system.