A Metro transit police officer in Washington, DC has been arrested on charges he attempted to provide material support to Islamic State, according to the US Justice Department.
In July, Nicholas Young, who lives in Virginia, sent codes for gift cards worth US$245 (NZ$342) to an FBI informant.
The gift cards were intended for mobile-messaging accounts that Islamic State uses to recruit its followers. Young believed the informant was an acquaintance of his who was working with the militant group, court records said.
The 36-year-old Young, who had worked for the transit authority since 2003, had been on the radar of US law enforcement since 2010, according to an affidavit in the complaint filed in federal court in Virginia on Tuesday.
Metro authorities said Young was fired immediately after his arrest on Wednesday.
In 2014, he met several times with an undercover agent with the Federal Bureau of Investigation as an eager recruit of Islamic State, according to the affidavit, and advised the agent about how to evade law enforcement as he left the United States to join the militant group.
"Metro transit police alerted the FBI about this individual and then worked with our federal partners throughout the investigation," said Metro general manager Paul Wiedefeld.
"Obviously, the allegations in this case are profoundly disturbing," Mr Wiedefeld said.