TAIPEI, Taiwan (AP) -- When Taiwanese security personnel detained a suspected spy for China at a top secret military base last month, they may have had a sense of deja vu.
Air force Capt. Chiang -- he was identified only by his surname -- was the fourth Taiwanese in only 14 months known to have been picked up on charges of spying for China, from which the island split amid civil war 63 years ago. While Taiwan's Defense Ministry did not disclose details of his alleged offense, his base in the northern part of the island hosts the air force's highly classified radar system and U.S.-made Patriot surface-to-air missiles, both vital to the island's aerial defense.
Chiang's arrest followed that of Maj. Gen. Lo Hsieh-che, who had access to crucial information on Taiwan's U.S.-designed command and control system, and civilian Lai Kun-chieh, who the Defense Ministry says tried without success to inveigle Patriot-related secrets from an unnamed military officer. A fourth alleged spy was detained on non-defense-related charges.
The cases show that China is seeking information about two systems that are integral to Taiwan's defenses and built with sensitive U.S. technology. A major breach could make Taiwan more vulnerable to Chinese attack.
Though relations between the two have warmed in recent years, Beijing has never recanted a vow to retake the island, by force if necessary.
Information about the U.S.-supplied defense systems could also help the People's Liberation Army understand other U.S. defenses. Taiwanese officials, however, say their systems are secure, and U.S. experts say American secrets will remain protected in any case.
The possibility that Taiwan might give up military secrets is certainly a worry for the U.S., its most important foreign partner.
Despite shifting recognition from Taipei to Beijing in 1979, Washington continues to sell the island sophisticated military equipment, and sees it as an element in a string of Asian defense relationships that stretches from South Korea to Australia. Any confirmed leak of U.S. defense secrets from Taiwan to China could undermine U.S. willingness to continue providing military equipment and technology to the island.
"We are concerned whenever this type of incident occurs," a U.S. defense official said in an email response to an Associated Press request for comment on the recent espionage incidents. "However, Taiwan has taken aggressive steps in the last year to protect itself from intelligence threats." The official spoke on condition of anonymity because of the sensitivity of the issue.
China and Taiwan have been spying on each other for decades, and U.S. intelligence agencies have also been active on both sides of the Taiwan Strait, including sharing sensitive mainland-related data with Taiwan. But the recent arrests represent a big upsurge in both the seriousness and quantity of Taiwan spy cases compared with previous years.
At the heart of the China's Taiwan espionage efforts are two systems with substantial U.S. technology -- the Lockheed Martin and Raytheon-built Patriot missile defense system and the Lockheed-designed Po Sheng command and control system.
The Patriot uses sophisticated radar to track incoming aerial threats, then launches high-performance missiles to bring them down. The Po Sheng network -- the Chinese name means Broad Victory -- allows Taiwan's army, air force and navy to exchange battlefield information in real time. That is a big advantage in coordinating responses to the attack China has promised if Taiwan ever moves to make its de facto independence permanent.
Defense expert Arthur Ding of Taiwan's Institute for International Relations said successful penetration of the Patriot system could wreak havoc with Taiwan's air defenses, a key component in turning back any future Chinese attack.
"China wants radar data so they can develop countermeasures," he said. "If you have this data you can jam the system or redirect its missiles."
Former Taiwan Deputy Defense Minister Lin Chong-pin said it is not surprising that China was targeting the Patriot and Po Sheng systems.
"These are several of our key capabilities which have been helped by the U.S.," he said. "They are the main obstacles to seizing Taiwan by force."
Deputy Defense Minister Andrew Yang agreed, calling Patriot and Po Sheng "a critical Taiwanese asset." But he told The AP, "The systems have not been compromised."
Beijing's biggest Po Sheng catch to date was almost certainly Maj. Gen. Lo, described by local media at the time of his arrest 14 months ago as the most effective Chinese spy on Taiwan since the 1960s, when a deputy defense minister was picked up in a sweep of communist agents.
Lo headed the army command's communications and information office, and according to Taiwan's defense ministry, he was recruited by the Chinese as a spy in 2004 when he was a military attache based overseas.
Taiwanese news reports say that Lo was arrested on the heels of U.S. surveillance, which determined that he had been recruited by a sultry female spy while serving in Bangkok. The reports said Lo had been blackmailed into providing Beijing with secrets involving electronic warfare and overall strategic planning.
The Defense Ministry says Lo's exposure to Po Sheng was limited. Last July he was sentenced to life in prison after being convicted on espionage charges.
Like Lo, Capt. Chiang had access to sensitive military secrets. Taiwanese news reports said he passed information about an early warning radar system through a Taiwanese businessman working in China.
Citing unidentified military sources, Taiwan's Apple Daily newspaper described the system as a joint Taiwan-U.S. air defense called "yellow net" that can track Chinese missiles launched at the island.
The defense ministry has acknowledged that Chiang had worked at a ground command center in northern Taiwan, without elaborating on what he did there.
The Apple Daily said officials concluded that a major motive for his alleged spying had been a desire to get money to impress his girlfriend with frequent visits to expensive nightclubs.
Two former U.S. government officials familiar with American defense sales to Taiwan said that despite some Taiwanese media reports, China's recent espionage activity on the island does not threaten the integrity of U.S. defense technology. They said Washington withholds sensitive information like sofnd equips highly classified electronic components with anti-tamper devices.
Still, more than just U.S. technology is at stake when Chinese spies target Taiwanese defense networks, one of the former officials said.
"How Po Sheng is used, the network layouts, what systems are integrated into the network and what are not, all this would be very useful for the Chinese to know," he said.
This kind of knowledge -- which would not necessarily compromise U.S. technology -- could help the Chinese pinpoint weaknesses in the island's overall defense alignment.
While insisting that China's espionage efforts had not undermined Taiwan's ability to defend itself, Yang, the deputy defense minister, said they showed that China has never let up on trying to steal Taiwan's most vital military secrets, despite Taiwan President Ma Ying-jeou's recent moves to try to lower tensions across the Taiwan Strait amid rapidly improving commercial and political relations.
"Nothing has really changed," Yang said. "Beijing has continued its espionage activities despite the improvement in ties."