A federal court order to detain Sinh Vinh Ngo Nguyen, 24, of Garden Grove states that arresting agents said he was due to travel to Mexico City by bus and then board a flight to Pakistan.
The order, which was made available online yesterday, also states Nguyen was holding a fake passport and had recently travelled to Lebanon and Syria to help the Free Syrian Army.
A message seeking comment from his attorney, Yasmin Cader, was not immediately returned.
Nguyen, who is also known as Hasan Abu Omar Ghannoum, attempted to work under the direction of al-Qaeda, according to an indictment returned last week.
The four-page document provides no details about the alleged terrorism act.
Nguyen's family has said he recently converted to Islam. He had a license to work as a security guard, though it was set to expire last month.
