Adel Abdul Bary, who is accused of helping to plan the 1998 bombings of US embassies in Kenya and Tanzania, has pleaded guilty in Manhattan Federal Court.
However, Judge Lewis Kaplan has not yet accepted a plea deal for the 54-year-old Egyptian that would see him serve 25 years in prison.
Adel Abdul Bary was charged with conspiracy to murder and the use of weapons of mass destruction, among other counts.
More than 200 people were killed when the US embassies in Nairobi and Dar es Salaam were attacked in August 1998.
Adel Abdul Bary was extradited from the UK in 2012 along with Mustafa Kamal Mustafa, also known as Abu Hamza.
He pleaded guilty on September 19 to several of the lesser of the more than 285 charges against him, including threatening to kill by means of explosive and conspiracy to murder US citizens abroad.
The judge has yet to rule on hundreds of other charges against him, including the murders of each person killed in the attacks and conspiracy to attack US national defense utilities.
On September 19, Adel Abdul Bary wiped tears from his eyes and shook his head as he made the plea, according to Reuters news agency.
Adel Abdul Bary admitted to using fax machines and phone calls to deliver messages of responsibility for the attacks to the news media, messages which included future threats against Americans.
Judge Lewis Kaplan said he wanted to hear further arguments before the more serious charges could be dropped.
He gave lawyers for both sides one week to submit arguments on why he should accept the deal, which would see Adel Abdul Bary sentenced to 25 years, potentially with credit for time served in the UK.
“You can well appreciate why I have questions in my mind,” Lewis Kaplan said.
Adel Abdul Bary would be permitted to withdraw the plea and proceed to trial if the judge rejects the deal.
Prosecutors told the hearing the US Attorney’s office felt the plea deal was “appropriate with regard to this defendant and the role he played in a much larger conspiracy”.
He was originally set to go on trial in November, alongside two others charged in terrorism cases.
Adel Abdul Bary was arrested in the UK in 1999. The US requested his extradition soon after, alongside radical preacher Abu Hamza and three others accused in a lengthy terrorism indictment.
Adel Abdul Bary is the father of the British man suspected to kill American journalists James Foley and Steven Sotloff.