On the other hand, it is possible the FBI acquired evidence of espionage, but may have compelling reasons for not disclosing it.

Among them: Exposing the details of how the FBI gathers evidence could hurt the ability of law enforcement to gather it in the future.

"There are very sensitive means of collection in the counterespionage realm" and the government "may have made a decision based on having to try a case in an open courtroom," said McGregor Scott, a former US attorney for the Eastern District of California.

Instead of espionage, the defendants are charged with conspiracy to act as agents of a foreign government while concealing that from the US government, a much easier case to prove. A second conspiracy charge of money laundering is based on payments to the defendants that allegedly originated at the headquarters of Russian intelligence.

This is hardly the first time suspected spies have been charged with something besides espionage. Mishandling classified information is one alternative to charging espionage when, for example, investigators have proof that someone removed classified information from government files but are unable to show that they gave it to a foreign power.