Many people who know P. Leonardo Mascheroni describe him as a maverick and a technology zealot. Now, the Justice Department will try to prove that he is dangerous, too — a man willing to sell atomic secrets in exchange for a chance to realize his dream.

Mascheroni, 75, is a nuclear scientist who has spent the 22 years since he left the Los Alamos National Laboratory trying to sell Congress, the scientific community, journalists — anyone who would listen, really — on his plan to build a giant laser for the achievement of nuclear fusion.

His plan earned respect and high-level endorsements, but the government chose a different path. Rather than give up, Mascheroni redoubled his campaign, sending out lengthy technical documents from his home in New Mexico to try to coax Washington to finance his laser.

"You’d get these fat FedEx packages," said Steven Aftergood, a security expert at the Federation of American Scientists.

As he was snubbed by Congress and federal experts, Mascheroni, a naturalized citizen who was born in Argentina, grew increasingly frustrated and bitter. He became known in Washington for veiled threats to take his atomic expertise abroad unless the government backed his laser plan. He seemed to think that he could bully the federal establishment into big spending, according to people on the receiving end of his missives.

"He came at you like a force of nature," recalled Matthew G. McKinzie, a former Los Alamos researcher who is now at the Natural Resources Defense Council, a private group in Washington. "He and his coterie of followers at Los Alamos believed his approach could provide a source of limitless, clean energy for humanity."

A 22-count indictment against Mascheroni, made public Sept. 17, quotes his wife, Marjorie, as saying that he would "make bombs" overseas "if they don’t listen to him in Washington." She has been charged as a co-conspirator, and both of them have pleaded not guilty.

Mascheroni’s world began to crumble in October, when federal agents raided his home in Los Alamos, hauling away his computers and hundreds of paper files. When reached by phone, he said at the time that he was suspected of treason.

Clearly rattled, Mascheroni declared his innocence. He defended his actions, including his interactions with the man who had represented himself as a Venezuelan contact but who Mascheroni by then suspected — correctly — was an informant for the Federal Bureau of Investigation.

"It was a way of getting attention in Congress" and trying to prompt hearings on Capitol Hill about the nation’s fusion program, he said of his decision to talk with the would-be foreigner.

"I told them, ‘If you don’t have hearings, I’m going to leave,"’ he said. "And they didn’t have hearings."

Mascheroni conceded that the man had promised him a lot of money but insisted that he wanted it simply "to make a big case. It was political leverage. That’s the bottom line."

"If I were a real spy, I would have left the country a long time ago," he added.

Federal prosecutors have charged Mascheroni with trying to sell nuclear secrets to Venezuela as part of a complicated scheme to have that country bring his laser to life. According to the indictment, he negotiated the deal in 2008 and 2009 with the undercover FBI agent, who paid him $20,000 out of an overall promise of nearly $800,000.

The laser that Mascheroni wants to build could provide — in theory — an unusual but potentially limitless source of energy for the world, and in particular for the United States, his adopted home.

In the October phone interview, Mascheroni drew parallels between his own idealism and that of J. Robert Oppenheimer, who led the Los Alamos team that built the world’s first atom bomb but later tried to slow the arms race.

"I have to put my science at a higher level," he said, emphasizing that his responsibilities to the scientific truth extended beyond duties to care for his family.

He suggested that he even might face a death sentence.

The couple have a son, who is a firefighter in Idaho, and a daughter, who is a lawyer in California. Marjorie Mascheroni lost her job at Los Alamos the day of the federal raid, and neither she nor her husband has agreed to an interview since they were indicted.

P. Leonardo Mascheroni, who is known as Leo, was trained at a physicist at the University of California, Berkeley. At Los Alamos, he was exposed to information on nuclear arms and worked on teams that sought to make energy advances.

In 1988, after nine years at the weapons lab, he left and embarked on a personal crusade to achieve what had eluded thousands of other scientists: a controlled version of nuclear fusion, the violent process that powers the sun, the stars and hydrogen bombs. His proposal — the use of a big laser — was considered among the most futuristic of the alternatives on the table.

Skeptical of federal plans for laser fusion, he promoted his own as cheaper, faster and far more likely to succeed. Its wavelength was much longer, and its blasts of concentrated light far easier to achieve. He dismissed resistance to his plan as an overzealous commitment to the status quo.

"It’s a cultural thing," he told The New York Times in 1988. "They don’t want to admit something different."

He won guarded approval. A Los Alamos panel led by Gregory H. Canavan, a respected senior scientist, found Mascheroni’s idea worth exploring. The main attraction, the panel said, was that his laser system might prove to be as little as one twentieth the cost of its rivals.

"It’s very important for our country to have this option for the future," Mascheroni said in a 1989 interview. "The other approaches are not going to work."

After leaving the weapons lab, Mascheroni toiled on his pet project without pay, relying on his wife to provide most of the family’s income. Her jobs at Los Alamos included technical writing and editing.

Mascheroni, meanwhile, persistently lobbied the Capitol for his laser plan. In 2003, for example, he wrote to Sen. John McCain, R-Ariz., who had just become chairman of the Senate Commerce Committee. The letter was 319 pages long, and Mascheroni sent copies to relevant experts outside of Congress.

Despite his rebel status and impolitic ways, he was often taken seriously. He won the backing of a former Central Intelligence Agency director, R. James Woolsey, who helped him promote his vision. Ultimately, however, the nation chose a more elaborate laser path.

That did not stop Mascheroni. The indictment against him describes clandestine meetings, tape-recorded conversations, confidential places for the transfer of documents and a pattern of false statements from Mascheroni and his wife to federal authorities.

As part of the plot, prosecutors say, Mascheroni would build a laser for producing energy — as he had always wanted to — and would throw in a plan for Venezuela that "could deliver a nuclear bomb in 10 years."

Lawyers in the case say that a trial may not get started until the middle of next year. If found guilty, Mascheroni and his wife could face up to life in prison.

As Mascheroni sits in a halfway house in Albuquerque awaiting trial, the rival laser that he criticized for so many years now looms over a small California town. The size of a football stadium, the $3.5 billion site is known as the National Ignition Facility. It is the world’s most powerful assemblage of lasers, their concentrated light like a tiny star. The 192 lasers fire in unison on flecks of hydrogen fuel smaller than a match head.

Skeptics dismiss facility as a colossal delusion that is squandering precious resources — $140 million a year — at a time of economic hardship. But scientists at the Lawrence Livermore National Laboratory, its home, insist that the laser is making great progress toward its goal.

Hugh E. DeWitt, a California physicist and veteran of the Livermore lab, suggested that Mascheroni, in approaching a man he thought was a Venezuelan agent, was probably overselling his bomb skills in an last-ditch attempt to bring his laser to life.

"He was never particularly interested in nuclear weapons," recalled DeWitt, who attended Mascheroni’s wedding. "His interest was in laser fusion."

But Mascheroni’s fixation on the giant laser transformed him into "a gullible nut," one who readily stumbled into the FBI’s trap," DeWitt said, adding: "He has dug his own grave."