Last July, as they drove to Dulles airport in a rented car, Kuo Tai-sheng reached over and stuffed a wad of cash in the shirt pocket of Gregg W. Bergersen.

That is the Federal Bureau of Investigation's (FBI) version of events, in any case. They had been tracking Mr. Bergersen, a Defense Department weapons policy analyst, and Mr. Kuo, a furniture dealer and naturalized US citizen living in New Orleans, for months, searching their hotel rooms, wiretapping phone calls, even secretly copying the contents of Kuo's computer hard drive.

According to conversation transcripts from a Justice Department affidavit unsealed Monday, Bergersen said, after receiving the money: "Now the other information I gave you, I'm very, very, very reticent to let you have it, because it's all classified." He continued, "But I will let you see it ... and you can take all the notes you want."

Bergersen, Kuo, and a third alleged conspirator, Yu Xin Kang, were arrested on espionage charges Monday. In a separate case, former Boeing engineer Dongfan "Greg" Chung was also arrested the same day and charged with illegally passing company trade secrets.

These cases suggest that Chinese espionage has become one of the most pervasive counterintelligence problems in the United States, say US officials and outside analysts.

Beijing's spying style is difficult to counteract, some experts say. China does not utilize a few, highly placed, and deeply embedded agents as the Soviet Union did. Instead, it employs a vast, decentralized network of Chinese and China-born students, business people, and scientists, and acquires information one small bit at a time.

"China's espionage activities are the single greatest threat to US technology and strain the US counterintelligence establishment," said Heritage Foundation analyst Larry Wortzel, chairman of the US-China Economic and Security Review Commission, at a Jan. 29 House hearing.

When it comes to charges of Chinese espionage, it is important to remember that the US has had some high-profile cases fall apart. In 1999, Los Alamos National Lab scientist Wen Ho Lee was charged with stealing nuclear secrets for China. In 2000, all but one charge was dropped. Many Chinese-Americans felt he was being singled out because of his ethnic heritage.

The charge sheets in the two new cases, however, contain details that paint pictures of specific and continued spying for the Chinese government.

Bergersen worked at the Defense Security Cooperation Agency based in Arlington, Va. He is charged with passing plans and information related to US military sales to Taiwan to Kuo and his associate Kang, a woman who worked for Kuo's furniture business.

According to US law enforcement, Bergersen passed some of this information on when the two met in Las Vegas last April. FBI agents later searched Kuo's hotel room and found unclassified US military documents. Kuo was allegedly in contact with a Chinese government contact the FBI describes as a "PRC official."

In the second case, Mr. Chung is a naturalized US citizen and a longtime Rockwell and Boeing engineer who worked on the space shuttle program.

Chung would receive specific requests from China for information related to his work, officials charged. He would take the relevant documents when he went home, according to an indictment originally brought by a grand jury last October.