By Jim Finkle and Joseph Menn BOSTON/SAN FRANCISCO (Reuters) – 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 Incs 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. Investigators are still gathering information on the scope of the cyber campaign, which the authorities believe is continuing.