A Chinese-born engineer was found guilty yesterday of conspiring to export US defence technology to Beijing – including data that would make it easier to detect submarines – as the FBI gave warning that Chinese spies had become the most active secret agents in America.

Prosecutors described Chi Mak, 67, as a brilliant sleeper agent who had been passing defence technology secrets to Beijing for more than 20 years. He will be sentenced on September 10 and faces up to 45 years in jail.

The Government accused Chi, a naturalised citizen who had emigrated to America in 1978, of copying thousands of pages of documents from his defence contractor employers and giving them to his brother, who passed them to Chinese authorities over a number of years.

In the past two years the FBI has arrested nearly 30 Chinese nationals or Chinese Americans in cases involving US technology.

The Government has also set up more than 400 investigations since 2000 involving the illegal export of arms technology to China.

Last year one accused Chinese spy, Ko-Suen “Bill” Moo, pleaded guilty to charges that he tried to buy military parts and weapons, including an F16 fighter jet engine and cruise missiles.

Chi, an electrical engineer for Power Paragon based in Anaheim, California, was arrested in October 2005 after agents stopped his brother and sister-in-law boarding a flight from Los Angeles to Hong Kong.

Investigators said that they found three encrypted CDs in their luggage that contained documents on a new quiet-propulsion system for submarines, a technology aimed at making them almost impossible to detect. The discs also contained data on powering navy ships that Power Paragon was developing for the Pentagon.

The Government said that after his arrest Chi confessed to having passed information to China for more than 20 years, although he said at his trial that the confession had been coerced.

His arrest came after the FBI had watched him for 18 months, using cameras, wire-taps and microphones hidden in his car and work cubicle.

FBI agents also said that they found a “wish list” of US military technology wanted by the Chinese. Behind Chi’s image as an unassuming, quiet workaholic, prosecutors painted a shadowy world of Chinese codenames such as “Red Flower” and “Autumn Orchid”.

Chi’s wife, brother and other relatives face trial on June 5. Prosecutors say that they will use his conviction to try to negotiate plea bargains with the family, who, like Chi, pleaded not guilty.

Chi was also found guilty of acting as an unregistered foreign agent, attempting to violate export control laws and making false statements to the FBI.

One of the government prosecutors, Craig Missakian, said after the verdicts: “To the extent that there is Chinese espionage going on in the United States, I hope [the conviction] sends a strong message.”

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