Nicholas Young was arrested by the FBI on Wednesday but was on law enforcement's radar since 2010, according to an affidavit released with Young's arrest.
There was no evidence of any threat to the DC Metro system. Young will make his first appearance in court later Wednesday, according to two law enforcement officials.
He would be the first police officer in the United States arrested and charged with supporting ISIS.
Young has been in contact with undercover law enforcement officers and informants since 2011 and was interviewed as early as 2010 about his relationship with a friend, Zachary Chesser, who pleaded guilty to supporting a foreign terrorist organization, according to court documents.
In addition, Young allegedly met several times with Amine El Khalifi before el Khalifi was arrested in 2012 for plotting to carry out a suicide bombing at the US Capitol building.
In 2014, Young also began meeting with an informant who convinced Young he was traveling overseas to join ISIS in Syria, according to the FBI. Once Young believed that had happened, the FBI began communicating directly with Young electronically, posing as the informant.
The core charge of material support came from Young allegedly buying gift cards to support ISIS through mobile messaging accounts. He sent the gift card codes to the person he believed to be in Syria in late July.
Young also traveled to Libya in 2011 and tried to go a second time, telling law enforcement he was working with rebels to overthrow Moammar Gaddafi, according to the court documents. Numerous times he allegedly expressed desires to stockpile weapons, attack law enforcement and warned others about being watched.
Young was a Metro police officer since 2003 but was fired Wednesday morning after his arrest, according to Metro Transit Police.
"This investigation began with concerns that were reported by the Metro Transit Police Department, and it reinforces that, as citizens, we all have a duty to report suspicious activity whenever and wherever it occurs," Metro Transit Police Chief Ron Pavlik said in a statement.
Metro General Manager and CEO Paul Wiedefeld called the case "profoundly disturbing," saying the agency has been working "hand-in-glove" with the FBI throughout the investigation.