In the furor over the investigation of Wen Ho Lee, the former Los Alamos National Laboratory scientist suspected of spying, the operating assumption of most people seems to be that where there is espionage there must be evidence of espionage. A corollary to this is that failure to produce such evidence is proof of either a bungled investigation or a rush to judgment -- in this case, a judgment that some say erroneously singled out Mr. Lee because of his race.

My take is completely different. I think what we are seeing is evidence that China may have succeeded in devising an espionage strategy that can, over time, consistently defeat our ability to investigate or prosecute spying offenses.

China does not normally pay an agent for information, request that the agent provide classified documents, use intelligence officers to elicit information from the agent or engage in clandestine activity like "dead drops" in the United States. This means that there are rarely the "smoking guns" that we have in other cases of espionage -- the unexplained bank deposits, the videotapes of a suspect leaving items in a hollow rock in the park.

China prefers to obtain its information a little bit at a time, by having its scientists and experts exploit individuals who are visiting China in the normal course of business. The problem for American investigators and prosecutors is not to determine whether someone under investigation has provided information to China, but to prove somehow that he told the Chinese three things when he was authorized to discuss only two, or perhaps four things instead of three.

This is very bad news for investigators and prosecutors, who have little they can fall back on. For example, a wiretap of a suspect's phone may provide some useful background or personality assessment information, but it is unlikely to yield instantly incriminating conversation of the "bring your documents and meet me in the park" variety. Unless a suspect for some reason is cooperative enough to confess, there is little chance that a case can be made against him. Even if it can be shown that the suspect provided information, it is likely to remain unclear whether this was deliberate or inadvertent.

The principle that the Chinese apply is simple: people will almost never commit espionage, but they will often enough be indiscreet -- sometimes perhaps to the point of making a major mistake -- if they can be put in the right circumstances.

In other words, China doesn't so much try to steal secrets as to try to induce foreign visitors to give them away by manipulating them into certain situations. Such circumstances can include appeals to altruistic feelings: "Scientific information should recognize no political boundaries." Or to ego: "Someone as knowledgeable as you must know a hundred things that would help our research immeasurably but would not hurt your country at all."

In addition, it has been common enough for the Chinese to arrange a grueling day of tourism for visitors, followed by an evening cocktail reception at which a graduate student might seek research assistance, in the process repeating a question that the visitor had previously been unwilling to answer when asked by a senior Chinese colleague. It also has been possible to simply embarrass valuable information out of a guest by being so frank and open with him about, say, China's neutron bomb design work that he will want to offer at least a helpful hint in return.

China's strategy has some inherent limitations. Since China seeks only a little information at a time and seeks it only when a visitor to China can be maneuvered into the right circumstances, its effort moves rather slowly. It could speed things up with a "dollars for documents" approach, but that is a much riskier strategy because of the evidence trail it leaves.

As far as I can see, we are mishandling this threat to our national security because we are focused on the wrong questions, looking at who stole what information, in the expectation that our search will uncover one or more major spies. While it's possible there are major spies out there, it's not very likely.

The United States should instead focus on where and how China is obtaining information. China's espionage methodology, not a particular spy, is the main threat. The fact that China apparently has managed to acquire significant information without paying money for it makes me suspect that the root problem is people making mistakes, rather than people committing espionage.

Thus our national strategy should be prevention, not prosecution. China's approach should be vulnerable to a systematic program intended to eliminate or steer clear of what might be termed "occasions for indiscretion" that confront American visitors.