U.S. officials say it is unlikely espionage charges will be brought against a physicist fired for security violations from Los Alamos National Laboratory in March because of lack of concrete evidence.
Analysts say one of the strengths of China's intelligence gathering is that its methods make building a prosecutable case very difficult.
China has denied stealing U.S. nuclear secrets and the American physicist, Wen Ho Lee, has denied passing on any classified information.
China does not operate in a way that leaves a clear trail such as exchanging money for secret documents, said Paul Moore, former FBI chief analyst for Chinese counterintelligence.
Instead, China gathers slow drips of information over a long period of time from American scientists having professional interaction with their Chinese counterparts, said Moore, now a consultant who left his 20-year FBI position in 1998.
"You don't have suspicious situations like somebody skulking around in the park," Moore said.
Without money and documents changing hands and no guilty activities in sight by foreign intelligence officers and American suspects, "then you don't have the makings for a good prosecution. You also find it very, very difficult to investigate the situation," Moore said.
China has developed tactics to extract information that is more subtle than a clear-cut case of espionage, Moore said. For instance, Chinese scientists may reveal to their visiting American counterparts projects that they are working on. The American scientist is then made to feel obligated to reveal something about his or her work, Moore said.
"So they are not actually trying to steal the information so much as to get these people to present it to them," he said. "You have a peer situation where the information is elicited through questions from bona fide researchers, not spies, not intelligence officers," he added.
"What the Chinese do is they found a way to get essentially good people to do bad things," Moore said.
"I don't know what happened with Wen Ho Lee, but that would be the way they would normally approach things," he added.
The public linking of Lee to suspicions of Chinese espionage, despite the lack of criminal charges, has led to some cries of foul justice and accusations of ethnic bias.
"I am distressed by the fact that he (Lee) has lost his presumption of innocence," said Steven Aftergood, senior research analyst at the Federation of American Scientists.
"For the rest of his life he is tagged as 'suspected spy' or 'accused spy' Wen Ho Lee. And even a real spy is entitled to better treatment in America -- the rights that he has are the rights that all of us have," Aftergood said.
The Justice and Energy departments are currently negotiating over what statements taken during the investigation can be declassified. Following the completion of that process, a decision could be made on whether or not to prosecute.
Since the spying scandal broke, the Energy Department which oversees the nuclear weapons research labs has installed new rules aimed at beefing up security, including more scrutiny of foreign visitors from countries deemed "sensitive."
The Energy Department is also launching a polygraph program which will require the lie-detector test be taken by employees in the most sensitive areas of the labs.
"A person sitting in a room with a bomb, guarding it, well we want to make sure that the person is stable -- not a spy, not a cocaine addict," Ed Curran, director of counterintelligence at the Energy Department, said.
Curran said it is his responsibility to make sure American scientists traveling to sensitive countries for professional reasons are briefed on what to watch out for.
One method used by China with Chinese-American scientists is to try evoking ties to their ethnic heritage, experts said.
"You don't want to go out and look at your grandmother's site, you don't want to go to your father's old village, (because) the next thing you know you're being asked different questions," Curran said. "Their (China's) purpose is to blur that line as much as they can, my responsibility is to bring it back in focus," he said.