Authorities in Chicago have charged 19-year-old Mohammed Hamzah Khan with an attempt to leave the United States and join Islamic State fighters in Syria.
He was arrested a month ago at Chicago's O'Hare International Airport.
Federal prosecutors said during a pretrial hearing Monday that his younger brother and sister were also arrested on October 4 as they tried to depart for Istanbul, Turkey.
The judge said Khan is a potential threat to the United States and ordered his continued detention.
Only Hamzah Khan was charged Monday with intent to provide material support to a foreign terrorist group, which carries a a maximum 15-year prison sentence.
His 16-year-old brother and 17-year-old sister were not charged, but prosecutor Richard Hiller said they also were passionate supporters of the Islamic State group.
Other attempts by US teens
Last month three girls from Colorado were arrested at Frankfurt Airport in Germany and returned to the United States, after their parents found out they had run away to join the Islamic State militants in Syria, and alerted the authorities.
The girls were traveling to Turkey by way of Germany. They were released after questioning.
Many foreigners who want to join Islamic State militants cross into Syria or Iraq through Turkey's porous borders with those countries.
Islamic State militants have taken control of large swaths of Iraq and Syria since March and the group is leaving death and destruction in its path.
The group's stated goal is to form an Islamic caliphate. Its rigid agenda and early military success resonate with many young disgruntled Muslims in the western countries.
CIA estimates that as many as 15,000 foreigners have joined the group's forces in Iraq and Syria.