SANTA ANA, Calif. — At the Chinese espionage trial under way in federal court here, there is a constant clattering from the far end of the front row. The sound comes from a rotating team of FBI typists racing to transcribe every word uttered, so the bureau’s headquarters in Washington can get a nightly report on the case’s progress.

They are just a few of the more than a dozen government representatives listening intently in the spectators gallery, many taking copious notes.

The FBI has good reason to be apprehensive about its prosecution of an electrical engineer who worked for decades in Southern California’s defense industry, Chi Mak, 66. The handful of Chinese espionage cases brought in recent decades have often ended with the government disappointed, or even mortified, at the outcome.

How the government will fare with Mr. Mak will not be known until after the jury gets the case next week. However, the parallels between his case and the prior ones are striking, and they illustrate the many challenges involved in combating Chinese intelligence gathering in America.

“The government has had great difficulty prosecuting Chinese cases,” a former FBI counterintelligence agent who testified for Mr. Mak’s defense, Ivian Smith, said in a recent interview. His comment was something of an understatement.

In 1981, the FBI confronted a Taiwan-born nuclear scientist, Gwo-Bao Min, at an airport as he prepared to travel to China. They suspected Mr. Min was relaying sensitive information to the Chinese from the government laboratory where he worked. He was carrying a notecard that discussed miniaturizing nuclear weapons, but prosecutors deemed the evidence insufficient to bring charges.

In 1991, the FBI came to suspect that a physicist who was also born in Taiwan, Peter Lee, was passing military information to China. The probe languished for a time, but picked up again after Lee traveled to China, where he lectured a Chinese group on submarine detection technology he was not authorized to discuss. He ultimately pleaded guilty to two felonies, disclosing classified information and making false statements to investigators. The Navy discouraged bringing some serious charges because a trial could call attention to the fact that some of the data had been declassified.

Lee was sentenced to a year in a halfway house. The lenient sentence prompted years of wrangling in Congress about whether investigators and prosecutors mishandled the case.

Neither the Min or Lee cases brought the FBI nearly as much chagrin as that of Katrina Leung. In 2003, Leung, who had received $1.7 million as an informant for the bureau over two decades, was arrested and charged, in essence, with being a double agent for China. While the alleged presence of a turncoat among the FBI’s so-called assets was worrisome enough, it also emerged that Leung had long-running sexual affairs with two veteran FBI agents, including her own handler at the bureau. A federal judge dismissed all charges against Leung after finding that prosecutors engaged in misconduct. While that ruling was on appeal, Leung pleaded guilty to one false-statements charge and a tax charge. She was sentenced to probation.

Investigators say the relatively diffuse methods employed in Chinese intelligence gathering pose a challenge to many prosecutions. While Russian and Eastern European spy agencies were known to rely on a few well-compensated recruits who employed cloak-and-dagger techniques to gather crucial secrets, China tends to seek a broad array of technology by relying on the sentiments and loyalties of Chinese-Americans and Chinese students in this country. One result is that the culpability of those involved can be harder to prove in court, especially beyond a reasonable doubt.

“You’re talking about lots of pointillistic dots you have to connect, which isn’t as easy to do as having a videotape of someone making a drop and getting a big chunk of money,” a lawyer who prosecuted Peter Lee, Jonathan Shapiro, said yesterday. “Everyone who’s ever prosecuted a case involving the People’s Republic of China has discussed that issue. What’s an open question is will a jury understand it?”

Lee argued, as Mr. Mak’s defense has, that he was engaged in the ordinary exchange of ideas among scientists. Both men also complicated their prosecutions by arguing that the information was in the public domain or not well-protected. “These trials … basically end up putting corporations and the government on trial for failing to have stringent enough controls over the data,” a former FBI counterintelligence executive, Edward Appel Sr., said.

One salient fact that has gone undiscussed at Mr. Mak’s trial is how the investigation of him began. A reporter for the Washington Times, Bill Gertz, wrote that the alleged family spy ring was fingered by someone in the Chinese security apparatus. Such a person could be an enormously effective prosecution witness, but, to protect intelligence sources and methods, he or she will probably never set foot in an American courtroom.

“The most incriminating facts about espionage are often not used at trial, including the intelligence that originated the investigation,” Mr. Appel said. “The Rosenbergs were executed generations before the full truth was revealed.”

One China case where the government had considerable success was that of a longtime CIA translator, Larry Wu-Tai Chin. Chin confessed in 1985 to working for a handler in China’s Ministry of State Security. However, the former CIA analyst said he was simply trying to improve relations between America and China. A jury convicted him of espionage, but investigators never got the chance to see him sentenced for his crimes. On February 21, 1986, Chin, who was facing a possible life sentence, hanged himself in a Virginia jail cell.