It's a separate case from the one involving nuclear secrets stolen from Los Alamos Weapons Lab and America's W-88 nuclear warhead. The neutron case has been under investigation for several years, and remains active and unsolved.
The U.S. has conducted tests, but there are no neutron weapons in the U.S. arsenal.
Sources tell CBS News it's further proof of China's aggressive - and successful - espionage efforts targeting America's nuclear weapons labs.
In the W-88 case, Energy Department officials allege that Wen Ho Lee, a government computer scientist fired for alleged security violations, was involved in passing nuclear secrets to China, including design plans for the sophisticated nuclear warhead.
Law enforcement sources have told CBS News they believe the real culprits are still unidentified and possibly still working at the weapons labs.
Congress is planning an investigation of the security breaches.
"Unquestionably there are problems that remain at the national laboratories that need to be addressed - and urgently so," said Rep. Christopher Cox, R-Calif., who has written a classified report on Chinese espionage.