"I was saddened to learn the news of Joe's death," Sen. Harry Reid, D-Nev., said in a statement. "He worked hard to help protect Nevada by stopping the construction of the dump at Yucca Mountain.

"His work on behalf of our state will not be forgotten," Reid said. "I extend my sympathy to his family."

Bob Loux, executive director of the Nevada Agency for Nuclear Projects, said Egan "was a fabulous lawyer and a talented guy, and he will be missed."

Allen Benson, a spokesman for the Department of Energy, said, "We are saddened by this news and extend our deepest sympathy to his family."

Egan's firm, then called Egan and Associates, was hired by the state in 2001. Between then and 2005, it initiated nine lawsuits against the federal government and sought to halt plans to bury high-level nuclear waste at the Yucca Mountain site, 100 miles northwest of Nevada.

A victory in a 2004 lawsuit invalidated a radiation safety standard for the repository. A new standard still has not been finalized, contributing to delays in the Yucca program.

More recently, the firm, now known as Egan, Fitzpatrick & Malsch, has been engaged in administrative debates before the Nuclear Regulatory Commission while preparing for repository license hearings at the agency.

During his illness, Egan over time reduced his public role as the state's advocate in favor of his two legal partners, Martin Malsch and Charles Fitzpatrick. Egan maintained a presence behind the scenes.

Fitzpatrick, who disclosed Egan's death, said a memorial service was being planned for May 22 in Naples. Other details were unavailable.

He leaves his wife, Patty, and two children.

Besides being an attorney, Egan was a former station-certified nuclear reactor engineer with degrees in physics, nuclear engineering and technology and policy from the Massachusetts Institute of Technology. He earned a law degree at Columbia University.

Before founding his firm, Egan was a partner at Shaw Pittman in Washington, D.C., and a senior associate at LeBoeuf Lamb Greene & MacRae in New York City. Both firms specialized in nuclear and public utility law and litigation.