He peers out from the photo in the classified file through heavy-framed spectacles, an owlish face with a graying beard and a half-smile. Saifullah Paracha, a successful businessman and for years a New York travel agent, appears to be the oldest of the 172 prisoners still held at the Guantánamo Bay prison. His dossier is among the most chilling.

In the months after the Sept. 11 attacks, Mr. Paracha, 63, was one of a small circle of Al Qaeda operatives who explored ways to follow up on the hijackings with new attacks, according to the classified Guantánamo files made available to The New York Times.

Working with Khalid Shaikh Mohammed, the 9/11 planner who in early 2002 gave him $500,000 to $600,000 "for safekeeping," Mr. Paracha offered his long experience in the shipping business for a scheme to move plastic explosives into the United States inside containers of women's and children's clothing, the files assert.

"Detainee desired to help Al Qaeda 'do something big against the U.S.,' " one of his co-conspirators, Ammar al-Baluchi, told Guantánamo interrogators, the files say. Mr. Paracha discussed obtaining biological or nuclear weapons as well, though he was concerned that detectors at ports "would make it difficult to smuggle radioactive materials into the country," the file says.