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(CNN) —  

George Aref Nader, who was a key witness in special counsel Robert Mueller’s Russia investigation, was arrested on child pornography charges Monday in New York, federal prosecutors announced Monday.

Nader was arrested upon arrival at John F. Kennedy International Airport for “transporting visual depictions of minors engaged in sexually explicit conduct.”

He previously pleaded guilty to the same charge in 1991, the Justice Department said.

If convicted, he faces a minimum sentence of 15 years in prison and a maximum of 40 years.

Nader was charged by prosecutors in Virginia, but made his initial appearance before a magistrate judge in Brooklyn on Monday. US Magistrate Judge Cheryl Pollack postponed a decision on Nader’s bail until Tuesday in order to give Nader’s attorney additional time to provide details of a proposed security arrangement that can accommodate what the attorney said is his client’s serious medical condition.

Pollack said if not for Nader’s health concerns she would have ordered him remanded to the Eastern District of Virginia.

“He has every incentive to flee,” she said.

Pollack ordered Nader to be detained overnight ahead of his bail hearing scheduled for Tuesday afternoon, according to the office of the US Attorney for the Eastern District of New York.

The Mueller report mentions Nader in nearly four dozen footnotes, including references to his interviews with special counsel investigators, and other materials, like his emails and text messages.

Investigators seized electronic devices from George Nader on January 17, 2018, according to the Justice Department. CNN previously reported that Nader was questioned by the FBI on that date, the first of several sessions with investigators from Mueller’s office. The warrants were issued in relation to a “matter unrelated to child pornography,” according to the FBI.

Nader’s arrest warrant was issued and a sealed criminal complaint was filed in April 2018, according to court records. Prosecutors sought to keep it confidential to prevent him from fleeing or tampering with evidence until he was arrested.

According to an FBI affidavit filed in support of the criminal complaint, the FBI found a dozen graphic pornographic videos of young boys on one of his iPhones.

Nader appeared in Brooklyn federal court in a beige button-down shirt and pants with what appeared to be a medical bracelet around his left wrist. He flew to the United States on Monday in order to attend an appointment with a doctor at Mount Sinai Hospital, according to both his attorney and a government prosecutor. He had open-heart surgery five weeks ago, his attorney said, and more recently experienced complications.

“He’s a sick man,” the attorney said.

His attorney has proposed he be transported by ambulance with armed security to the Eastern District of Virginia, an arrangement his lawyer said Nader himself would pay for.

Nader helped the Mueller investigation by providing multiple interviews and testifying before the grand jury. The Mueller report said “all but one” of his interviews were conducted “under a proffer agreement” where he could provide information with some legal protection.

He is cited extensively in the portions of the report about the Seychelles meeting in between Trump supporter Erik Prince and a Russian banker with close ties to Russian President Vladimir Putin. His testimony about his role in the secret January 2017 liaison undercut Prince’s claims that the meeting was a random encounter and that it was not coordinated with anyone from the Trump transition.

CORRECTION: This story has been updated to reflect that the sealed criminal complaint against Nader was issued in April 2018. Investigators seized electronic devices from Nader in January 2018.

CNN’s Brian Vitagliano, Katelyn Polantz, Kara Scannell and Eli Watkins contributed to this report.