The FBI said most of the “intrusions” are not public knowledge and cautioned that it does not know exactly how many computers have been compromised, according to the memo.
The FBI said the hacking that began last year exploited a flaw in Adobe software to access the government systems. It also left pathways for the hackers to easily gain re-entry, which has happened as recently as last month.
The memo cited break-ins to the Health and Human Services Department as well as the Energy Department. The memo said other agencies could have been affected as well.
One previously disclosed incident involved British citizen Lauri Love, who was arrested last month and charged with hacking into a number of federal agency computer systems using the technique.
The government said Love was accused of working with two partners in other countries who had not been charged. Internal emails from the Energy Department show 100,000 people associated with the department had some personal information stolen, according to Reuters.
Separate instances of alleged Anonymous hacking have sprung up this year. In July, a Twitter handle @OpLastRestort, claiming to be affiliated with Anonymous, posted outdated email login passwords for congressional staffers.
At the time, the Senate Sergeant at Arms said the hackers had gained access to an outside vendor but said the Senate computers were safe.