WASHINGTON -- The Federal Bureau of Investigation earlier this year launched a nationwide operation targeting white supremacists and "militia/sovereign-citizen extremist groups," including a focus on veterans from Iraq and Afghanistan, according to memos sent from bureau headquarters to field offices.

The initiative, dubbed Operation Vigilant Eagle, was outlined in February, two months before a memo giving a similar warning was issued on April 7 by the Department of Homeland Security.

Disclosure of the DHS memo this week has sparked controversy among some conservatives and veterans groups. Appearing on television talk shows Thursday, Homeland Security Secretary Janet Napolitano defended the assessment, but apologized to veterans who saw it as an accusation.

"This is an assessment of things just to be wary of, not to infringe on constitutional rights, certainly not to malign our veterans," she said on NBC's Today Show.

The documents outlining Operation Vigilant Eagle cite a surge in activity by such groups. The memos say the FBI's focus on veterans began as far back as December, during the final weeks of the Bush administration, when the bureau's domestic counterterrorism division formed a special joint working group with the Defense Department.