"I plead not guilty to all the counts," Abdullah said during his arraignment. He was ordered held without bail. He will return to court Jan. 6.
In preparation for the attack, the indictment alleges that Abdullah obtained pilot training in the Philippines, researched security on commercial airliners and how to breach a cockpit door, and also researched information about the tallest building in "a major U.S. city," which the indictment did not identify.
"This chilling callback to the horrific attacks of Sept. 11, 2001, is a stark reminder that terrorist groups like al-Shabaab remain committed to killing U.S. citizens and attacking the United States," acting U.S. Attorney Audrey Strauss said.
Abdullah allegedly worked at the direction of a senior al-Shabaab commander who planned the 2019 attack on a Nairobi hotel that killed 21 people, including a U.S. citizen who had survived the Sept. 11 attack.
Abdullah obtained a pilot's license in the Philippines and sought information about how to obtain a U.S. visa, the indictment said.
"Nearly 20 years after the 9/11 terrorist attacks, there are those who remain determined to conduct terror attacks against United States citizens. Abdullah, we allege, is one of them," the FBI's Bill Sweeney said.
In a statement to ABC News, a spokesperson from All Asia Aviation Academy, where Abdullah was enrolled, said, "As a student, Mr. Abdullah displayed no unusual behavior and during his period as a pilot trainee of almost 1 year, came in and out of the country without raising any red flags, including the regular compliance documentation that all students had to undergo with the different government offices here in the Philippines."
"His arrest and involvement with such a terrible organization after the investigation results were disclosed to us definitely came as a surprise. We're glad that he was taken into custody," they continued, adding that the school is cooperating with officials in the ongoing investigation.