PASADENA, Calif. — A Chineseborn electrical engineer who worked for a California-based defense contractor for more than two decades, Chi Mak, was convicted yesterday of conspiring to send China information on quiet propulsion systems for U.S. Navy ships and submarines.

After deliberating for 2 1/2 days, a federal court jury in Orange County, Calif., returned guilty verdicts on all five counts Mak faced, which included attempting to export controlled defense articles without a license, acting as an unregistered agent for Beijing, and lying to the FBI.

“We’re tremendously disappointed,” an attorney for Mak, Ronald Kaye, told The New York Sun. “We believe our client is innocent. The government’s case emphasized fear and was focusing on burning submarines on fire and sailors dead on the bottom of the ocean. The jury got scared, I think.”

The verdicts were a major coup for the bureau and the Justice Department, which have had serious difficulty prosecuting cases against alleged spies for China. Setting aside charges of economic or commercial espionage, the last successful prosecution of a spy for China was in 1986. A spokesman for the prosecution, Thomas Mrozek, declined to comment, citing a trial planned for later this month against Mak’s wife and three other relatives.

Mak, 66, a naturalized American who migrated here in 1978, was a longtime employee of Power Paragon of Anaheim, Calif. During his trial, which lasted about six weeks, federal agents detailed intense audio, video, and physical surveillance they conducted on Mak and his relatives between early 2004 and October 28, 2005.

That night, Mak’s brother and sister-in-law were arrested at Los Angeles International Airport as they prepared to board a flight a flight for Guangzhou, China. In the couple’s hand luggage, authorities found a disk that contained encrypted documents about quiet electric drive technology and ultrafast electrical switches.

One challenge the government faced from the beginning of the case was that the materials found on the disk were not marked as classified or treated as such by Mak’s employer. Indeed, Mak’s defense team contended that the papers on the disk were in the public domain because they were presented at conferences attended by foreigners and, in some cases, were available for purchase over the Internet.

The most damaging evidence prosecutors had were indications of cloak-and-dagger subterfuge by Mak and his family. Mak and his brother both had code lists in their homes that featured a series of words and phrases, such as “supermarket” for “good material.” Mak also had documents prosecutors described as shopping lists for technology Beijing was seeking.

Mak said he got the code from a niece who wanted to use it communicate about an ailing elderly relative in China. Testifying in his own defense, he called the code “ridiculous.” Nevertheless, he acknowledged that he copied the code out and gave it to his brother. Defense lawyers said the alleged shopping lists were of no significance because it is commonplace for academics and scientists to receive such feelers from China.

Agents from the FBI and the Naval Criminal Investigative Service said Mak confessed in an interview to sending sensitive defense technology to the Chinese government for two decades. At trial, Mak denied ever making such a statement and insisted that he was engaged in legitimate technological exchange. His lawyers noted that the alleged confession was not taped and that the agents’ notes did not reflect some of the purported admissions.

Mak conceded that in an initial interview with law enforcement he did not tell the truth about his travels to China and his relatives there. He said he felt pressure from the agents and from being arrested at him home in his bedclothes. “They pushed me that night,” Mak complained on the witness stand.

One of the defense’s strategies was to suggest that Mak may have been duped by his brother, an engineer for a Hong Kong-based television channel, Phoenix. There was evidence that the brother lied about his travel plans. In addition, the encryption took place at the brother’s home and the parties disputed whether Mak was aware of it.

In theory, Mak could be sentenced to up to 45 years in prison. However, his actual sentence is likely to be shorter. Some of the export charges typically bring a sentence of about four years.

An export attorney who consulted with the defense, Robert Burns, said he was not surprised by the outcome. However, he asserted that Mak has a strong argument to make on appeal that his trial was tainted by charges pertaining to documents in the public domain. “Even if you encrypted public domain documents, it wouldn’t turn them into controlled technical data,” he said.

“The verdict will have negative ramifications on the sharing of knowledge,” the defense attorney, Mr. Kaye, said. He also said it would lead Chinese-Americans to conclude that their actions will be viewed with more suspicion than those of other scientists and academics.