(CNN) -- The convicted mastermind of the 1993 World Trade Center bombing wants nearly two decades of communication restrictions lifted, arguing he's no longer a threat to national security, his lawyer said Sunday.
Ramzi Yousef has been locked away in solitary confinement at the federal "Supermax" prison in Florence, Colorado, since 1998. A 15-page list of rules sets limits on his contact with relatives, lawyers and other inmates. He can read books and watch television, but newspapers and magazines are censored to keep him from receiving messages planted in classified ads or letters to the editor.
Now 44, Yousef "no longer should be considered a national security threat," his lawyer, Bernard Kleinman, told CNN. "If the government feels that he is, they should provide some reasonable basis that they can corroborate as to why he is a continuing national security threat."
U.S. District Judge Kevin Duffy, who sentenced Yousef to life plus 20 years, called him "a virus that must be locked away." He was arrested in 1996 in a plot to bomb U.S. airliners in Asia, and he's the nephew of Khalid Shaikh Mohammed -- the accused mastermind of the 2001 attacks on New York and Washington that killed nearly 3,000 people and brought down the World Trade Center.
He went to court in 2011 to ask that the Justice Department's "special administrative measures" that restrict his contact be lifted.
Kleinman said that other than the al Qaeda defendants facing trial at the U.S. prison camp at Guantanamo Bay, Cuba, "there's nobody who's treated as a greater national security threat in the federal prison system than Yousef."
The Justice Department, which has kept those restrictions on Yousef since 1997, argues that Yousef is trying to find a back-door route out of solitary. Kleinman said getting out of solitary could be "the effective end result" of Yousef's case, but it's not the immediate goal.
"The goal is to find out why he is subject to these special conditions," he said.
In a 2011 ruling, Duffy wrote that while Yousef was awaiting trial, he had been trying to collect urea -- a component of urine -- and a type of watch that had been used in a detonator in the airline bomb plot. But Kleinman said that while his client was a smart man, "That doesn't mean that he could turn lead into gold."
"The federal government has viewed for years that he has this almost mystical ability to fabricate weapons or bombs almost out of thin air," Kleinman said. "There's never been any evidence of that whatsoever."
Duffy, who had sentenced Yousef to prison, referred the case to a judge in Colorado, where it's now pending. In January, the Justice Department argued the case should be dismissed on procedural grounds.