In a memo it distributed this week, the FBI warned that a breach of US government systems was “a widespread problem that should be addressed,” according to a Reuters report.

Activist hackers linked to the collective known as Anonymous have secretly accessed U.S. government computers in multiple agencies and stolen sensitive information in a campaign that began almost a year ago, the FBI warned this week.

The hackers exploited a flaw in Adobe Systems Inc’s software to launch a rash of electronic break-ins that began last December, then left “back doors” to return to many of the machines as recently as last month, the Federal Bureau of Investigation said in a memo seen by Reuters.

The memo, distributed on Thursday, described the attacks as “a widespread problem that should be addressed.” It said the breach affected the U.S. Army, Department of Energy, Department of Health and Human Services, and perhaps many more agencies.

The personal information of over 100,000 employees, family members and others associated with the Department of Energy was amongst the stolen data, according to the Reuters report.

According to officials, the hacking is linked to another case, in which a UK man was charged in connection with hacking into multiple US government computer systems, causing personal data of US military personnel to be compromised and millions of dollars in losses.  Lauri Love is alleged to have infiltrated computer systems at the Department of Energy, Army, and the Department of Health and Human Services, among others.

The FBI wrote, “It is unknown exactly how many systems have been compromised, but it is a widespread problem that should be addressed,” Reuters reported.  The memo advised system administrators on steps to take to determine whether or not their systems have been compromised.

Read the full report from Reuters.