A former CIA officer who pleaded guilty to an espionage conspiracy with China was sentenced Friday to 19 years in prison.

Jerry Chun Shing Lee was a spy who significantly damaged national security, intelligence officials said.

Sources familiar with the case told NBC News that Lee is suspected of funneling information to China that caused the deaths or imprisonment of approximately 20 American agents, in one of the worst intelligence breaches in decades.

Lee, 55, appeared in federal court in Alexandria, Virginia, on Friday. He previously pleaded guilty to conspiracy to commit espionage. But prosecutors and defense lawyers disagreed about the extent of the crime.

Prosecutors said Chinese intelligence officers gave Lee more than $840,000 and that Lee likely gave them all the information he had from a 13-year career as a CIA case officer.

Defense lawyers said the government never proved that the money came from China or that Lee ever carried out any plans to deliver government secrets. They asked for a 10-year sentence.