NEW YORK (AP) — A lawyer for the mastermind of the 1993 World Trade Center bombing asked a federal appeals court Wednesday to let him investigate why the government insists his imprisoned client is still a danger to America.

Attorney Bernard Kleinman wants the 2nd U.S. Circuit Court of Appeals in Manhattan to let him gather facts from prosecutors about why Ramzi Yousef remains held under the strictest security measures 15 years after they were first imposed. The three-judge panel did not immediately rule.

"I think it's just plain unfair," Kleinman said outside court. "Most of the terrorists he knew are either dead or in jail."

Assistant U.S. Attorney Nicholas Lewin argued for the government that the case belongs in Colorado rather than Manhattan because Yousef is challenging his prison conditions. Yousef, 44, is in solitary confinement at the so-called Supermax prison in Florence, Colo.

Yousef was sentenced to life in prison after he was convicted in the February 1993 attack that killed six people and injured hundreds of others and in a plot to bomb 12 American airliners over the Far East. He fled to the Middle East the night of the bombing and was captured two years later in Pakistan, where he was turned over to the FBI. He now lives in a 7-foot by 11-foot cell with a radio, a television, a desk, a toilet and a shower, Kleinman said.

Since 1997, he has been subjected to special administrative measures reserved for prisoners who are believed to be a continuing threat to the country. Kleinman said he is challenging the grounds under which the government continues to impose the severe restrictions on Yousef.

The restrictions prevent Yousef from communicating with other prisoners and only allow him to meet with his lawyer, Kleinman said. For those visits, the last of which occurred in October 2010, Yousef is shackled and forced to communicate through Plexiglass.

Yousef is a nephew of Khalid Sheik Mohammed, who has conceded in a written statement that he directed the Sept. 11, 2001, terror attacks.

Kleinman said Yousef's prison conditions were tightened even more after the Sept. 11 attacks so that he was no longer housed near notorious criminals including Unabomber Ted Kaczynski and Oklahoma City bombing conspirators Terry Nichols and Timothy McVeigh. McVeigh was executed while Nichols is serving life in prison.

At his 1998 sentencing, Yousef defiantly proclaimed: "Yes, I am a terrorist and am proud of it."

Kleinman said he knows his client is "not a sympathetic character" and that there are "legitimate hard feelings against him."

But he added that he hopes to eventually get the severe prison conditions loosened so that Yousef can have fewer restrictions on his communications with family, including his wife and two daughters.

"The human interaction is really a big issue for him," Kleinman said. "He recognizes he'll be in jail the rest of his life."