The Justice Department’s unusually tough stand against a California aerospace engineer who kept a trove of classified information at his home is prompting puzzlement and speculation about whether something more nefarious was afoot or whether prosecutors are heralding a new era in the treatment of those who are cavalier with America’s secrets.

Abraham Lesnik, 62, of Los Angeles, pleaded guilty last week to a single felony count of unauthorized possession of classified information, which he took home from his job at a Boeing facility in El Segundo, Calif. Lawyers and other experts who track the government’s actions in such cases were taken aback by the prosecution’s indication that it may seek a prison term of more than five years for Lesnik, who held a top secret clearance and specialized in anti-missile technology.

“Sixty-three months just for possession of classified is over the top. There must be something else going on here,” a former top official in the Justice Department’s Criminal Division, Robert Litt, said. “To the extent the government is saying everyone who improperly stores classified information deserves that kind of a sentence, that is ludicrous and is patently inconsistent.”

Lesnik, who holds a doctorate in physics from the University of Chicago, pleaded guilty to having 11 classified documents at his home in the Valley Village neighborhood of Los Angeles. One of those was “top secret” and pertained to “national defense satellite threat mitigation.” However, prosecutors said in a statement that Lesnik had “a very large number of classified documents” that he took from work using a “thumb drive.”

“It is of great concern to the government any time an individual violates the rules governing possession of classified information,” a Justice Department spokesman, Dean Boyd, said. The prosecution is expected to argue that the scientist repeatedly flouted the rules and put vital national secrets at risk.

In court, Lesnik said convenience was his motive. “I brought classified documents from my workplace to my home so I could work on them,” he said, according to the Associated Press. His defense said it plans to seek probation and no jail time.

An attorney who helps private-sector employees obtain and retain security clearances, Sheldon Cohen, said the felony charge and threat of a long prison term were virtually unheard-of for acts driven simply by arrogance. “Even if someone doesn’t care about the rules, it’s draconian,” he said. “Nothing like that ever resulted in a criminal prosecution.”

The charge Lesnik pleaded guilty to is part of the Espionage Act. However, he was not charged with espionage, and there is no mention in the plea agreement about him working for a foreign state. “If it was intended to be turned over to a foreign government, they would charge him with that. There’s not a word about that,” Mr. Cohen said.

In 2006, ABC News reported that investigators on the case had concerns “as to whether classified data ended up in the hands of unauthorized individuals, including foreigners.” Plea papers indicate that Lesnik may have failed to declare a foreign bank account, but they do not say where.

High-level government officials who fall into prosecutors’ cross-hairs for mishandling classified information rarely get jail time. In 2005, a national security adviser to President Clinton, Samuel Berger, received probation after pleading guilty to a misdemeanor for removing and destroying copies of a highly classified report he reviewed at the National Archives. In 2001, Mr. Clinton pardoned a former director of central intelligence, John Deutch, as he was about to plead guilty to a misdemeanor for having classified information on home computers hooked up to the Internet.

Prosecutors seeking a tough sentence for Lesnik are expected to cite the case of a National Security Agency analyst, Kenneth Ford Jr., who was convicted by a jury of taking two boxes of classified documents home on his last day of work. Ford, who blamed an FBI informant and suggested a conspiracy, was sentenced to seven years in prison.

Another puzzling aspect of the Lesnik prosecution is the sheer amount of effort the government has invested. Soon after Lesnik sued Boeing in 2006 in a dispute over private information on a laptop the company confiscated, the scientist’s home was put under rather obtrusive surveillance by the FBI. The close watch by multiple technicians in unmarked cars continued on and off for more than a year, according Eric Longabardi of ERSNews.com. “Either this is the biggest spy case in American history or this is the biggest waste of time and money in American history,” he said.

Lesnik’s attorney, Marc Harris, declined to comment for this article. A woman who answered the phone at Lesnik’s home said he was not available.

The scientist, who was fired from Boeing last year, seems to have been a backer of an unsuccessful libertarian candidate for the Republican presidential nomination, Rep. Ron Paul of Texas. “My name is Abe and I am a retired aerospace engineer. I support Ron Paul … on reducing govt spending, taxes, out of Iraq, and less foreign military aid,” Lesnik apparently wrote on a Web site, Meetup.com. About skills he could contribute, Lesnik wrote, “Speak german and yiddish. Good with math, computer, carpentry, electrical work, plumbing, auto repair.”

A Justice Department press release referred to the Boeing contract Lesnik worked on simply as one for “a component” of the Defense Department. However, a court document obtained by The New York Sun indicates that the project was for America’s secretive spy satellite agency, the National Reconnaissance Office. Congressional oversight committees were to be notified about Lesnik’s case, the papers said.

A lawyer specializing in classified information disputes, Mark Zaid, said the Justice Department would have trouble keeping up if it brought criminal charges against everyone who took secret or even top secret information home without permission. “They’re going to have on hand a serious caseload,” he said. “That concerns me greatly. You’re going to end up with selective prosecutions.”