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.
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.http://www.bbc.com/news/world-us-canada-36968052I bet he gets his job back.
90% of life is "showing up"