"I think it's likely there will be additional charges," Knox said at a pretrial hearing in federal court in Brooklyn. "The investigation continues."
The prosecutor didn't specify the charges, or say whether the indictment would name more defendants. He called the evidence in the case "voluminous."
Zazi, who sat motionless at the defense table during the 15-minute hearing, was ordered to return to court on Feb. 16.
"He's in fairly good spirits considering his situation," defense attorney Michael Dowling said afterward.
Authorities allege that Zazi, after receiving instruction on explosives at an al-Qaida training camp in Pakistan, bought beauty supplies in a Denver suburb to make peroxide-based bombs. They say he urgently tried to mix up explosives in a hotel room in early September, and then drove to New York to carry out an attack, possibly on the New York transit system.
Investigators, who had Zazi under surveillance, secretly searched his rented vehicle in New York on Sept. 10 and found a laptop computer with bomb-making instructions, authorities said.
Fearing he was being watched, Zazi flew back to Denver on Sept. 12, authorities said. He was arrested a week later.
Authorities have said that they believe other suspects were in on the plot. Two other men, including Zazi's father, have been charged with lying to the FBI, but no one else has been charged with terrorism.