as they left, bergersen just couldn't stop talking. >> bergersen: but i...ery careful to keep my tracks clean. >> kuo: of course. of course. >> bergersen: and no... no fingerprints. it's just like these documents... >> kuo: gotcha. >> bergersen: ... no fingerprints. i can't afford to lose my job. >> pelley: later, kuo left the u.s. for beijing. but while he waited for his flight, federal agents got into his bags, photocopied his handwritten notes, and put them back. kuo's notes matched the secret document on the right. but john slattery, who oversaw the case for the f.b.i., told us the bureau didn't make arrests until six month later. but, i mean, this is drop-dead evidence, and espionage is occurring. why didn't you arrest them sooner than that? >> slattery: well, these... these investigations are tremendously complex and tremendously difficult to begin with. >> pelley: the department of defense wants you to stop it right away. >> slattery: please-- sooner than later. but the f.b.i. says, "well, listen, we want to make sure we can sustain a conviction here.