Emails sent by officials at DHS, which oversees border security and defense against hacking, were monitored by the hackers as part of the sophisticated series of breaches, three people familiar with the matter told Reuters Monday.
The attacks, first revealed Sunday, also hit the U.S. departments of Treasury and Commerce.
Technology company SolarWinds, which was the key steppingstone used by the hackers, said up to 18,000 of its customers had downloaded a compromised software update that allowed hackers to spy unnoticed on businesses and agencies for almost nine months.
The United States issued an emergency warning on Sunday, ordering government users to disconnect SolarWinds software which it said had been compromised by “malicious actors.”
That warning came after Reuters reported suspected Russian hackers had used hijacked SolarWinds software updates to break into multiple American government agencies, including the Treasury and Commerce departments. Moscow denied having any connection to the attacks.
One of the people familiar with the hacking campaign said the critical network that DHS’ cybersecurity division uses to protect infrastructure, including the recent elections, had not been breached.
DHS said it was aware of the reports, without directly confirming them or saying how badly it was affected.