Two men arrested at New York's John F Kennedy Airport have been charged with conspiring to commit an act of international terrorism.
US authorities say they intended to join a jihadist group in Somalia and kill American troops overseas.
Mohamed Mahmood Alessa, 20, and Carlos Eduardo Almonte, 24, were held before they could board separate flights to Egypt.
They were due to appear on Monday in a federal court in Newark, New Jersey, on charges of conspiring to kill, maim, and kidnap persons outside the United States by joining al-Shabab, a militant group based in Somalia and linked to al-Qaeda.
Al-Shabab was designated a terrorist group by the US in 2008.
Conversations secretly recorded
The BBC quotes US justice officials as saying that state and federal law enforcement agents have been investigating Mr Alessa and Mr Almonte, both of New Jersey, since 2006.
A New York Police Department undercover officer is said to have recorded conversations with the men - both of them reportedly US citizens - in which they spoke about killing Americans.
They are also alleged to have watched online footage of attacks on US troops and suicide bombings, and practised for combat at paintball facilities and by using virtual-reality games.