A veteran police officer with one of the nation's most prominent transit systems has been arrested for allegedly trying to help ISIS, ABC News has learned exclusively.Authorities believe that while working for the Metro Transit Police Department near Washington, D.C. - a community he swore to protect - the officer was also trying to assist ISIS operatives find more ways to communicate in secret, sources told ABC News.The officer allegedly purchased technology-related items to send to the ISIS operatives so they could evade authorities when contacting each other, according to the sources.Instead of allegedly engaging with true ISIS associates, however, the officer was actually in touch with the FBI 's Joint Terrorism Task Force in Washington, which has been conducting a lengthy undercover investigation into the officer.There was "no pending threat to the D.C. transportation system," one source emphasized.The officer, a 12-year veteran of the transit police force, was arrested this morning. He lives in the Virginia suburbs of Washington.An arrest usually indicates charges have been filed in some form, but it's unclear when or how charges would have been filed in this case. It's also unclear when the suspect will be appearing in court, though terrorism suspects often have an initial court appearance within hours of their arrest.In the past three years, more than 100 Americans have been charged with trying to join ISIS or are suspected of supporting the group in some other way.