A police officer with the Washington transit system has become the first US law enforcement officer to be charged with supporting the Islamic State, accused of trying to send financial help to the group after advising a friend on how to travel to Syria to join it.

In court papers filed Tuesday and made public Wednesday, federal law enforcement officials charged the officer, Nicholas Young, with attempting to provide material support to a terrorist organization.

The charge is based on the allegation that Young bought gift cards worth $245 and sent their code numbers to someone he believed had joined ISIS in Syria, to help the group pay for mobile phone messaging with its supporters in the West.

The documents state that agents had been shadowing Young for almost six years, that he went to Libya twice in 2011 to aid a rebel group fighting Moammar Gadhafi, and that he had associated with two people convicted in 2012 on terrorism charges: Amine El Khalifi, who pleaded guilty to plotting a suicide bombing at the US Capitol; and Zachary A. Chesser, who admitted to trying to join Al Shabab, a Somalia-based Islamist terror group, and to threatening violence against the creators of the television show “South Park.’’

Young, a US citizen who lives in Fairfax, Va., joined the Washington Metropolitan Transportation Authority’s police force in 2003. The authority, which said he was dismissed after his arrest Wednesday, would not say where he was assigned or what kind of work he did.