The Federal Bureau of Investigation (FBI) has released surveillance videos of some of the Russian spies arrested last year in the US.
Using spy tools like invisible ink and hidden messages in online photos, the bureau's investigators built up a detailed file of the spies as they passed information and money.
But some intelligence officials worry the documents could give too much away and alert would-be culprits of tell-tale signs they are being watched.
Al Jazeera's Alan Fisher reports from Washington.
