>> reporter: a breach so large investigators are not sure how many government agencies are compromised. preliminary evidence points to vladimir putin's government being behind the attack. does the hack have the signature of russian intelligence? >> this definitely looks like the russians. it was skillful, targeted, advanced. >> reporter: the espionage campaign breached secure networks at the treasury and commerce departments and possibly homeland security, as part of a months-long effort to infiltrate u.s. agencies and private businesses. the hackers exploited solar winds, a software program used to monitor computer networks potentially affecting 18,000 companies and government agencies. this march, hackers latched on to a security update to burrow in. chris krebs, fired last month as the head of the government's cyber security agency called it, "a pretty large-scale hack." >> reporter: what makes it stand