From The RAND Corporation:

Dozens of young Americans like Nguyen have attempted to join overseas jihadist groups in the past several years, raising special concern among counterterrorism officials that they might bring the fight home with them when they return. The threat was punctuated with gunfire earlier this year, when two French brothers—Chérif and Saïd Kouachi—stormed the Paris offices of the news magazine Charlie Hebdo; both had reportedly trained with groups in Yemen and then slipped back into French society.

A RAND analysis by internationally renowned terrorism expert Brian Michael Jenkins of more than 100 cases found that almost all of the American jihadists who went overseas ended up dead or landed in the same place as Sinh Vinh Ngo Nguyen. Brought down by his trusted confidante, who was really working undercover for the FBI, Nguyen admitted in court that he was trying to get to Pakistan to help train al Qaeda fighters. He was sent to prison for 13 years.