The Department of Homeland Security documents an employee accidentally left behind on a commercial airplane last week contained "old" information, DHS Deputy Secretary Elaine Duke said Wednesday.

"It's old information. It's what we tend to call a hot wash of what we see and what we're looking forward to," Duke told Senate Homeland Security and Governmental Affairs member Doug Jones, D-Ala., during a roundtable discussion.

"The actual leaving of the documents — we'll be handling under a personnel matter similar to anything else that is a breach of our responsibilities of our employees," she added.

A CNN employee found "Official Use" documents on a plane that had detailed the federal government's planned response to a hypothetical biological attack at a Super Bowl game.

The documents were dated December 2017. An employee found them in the backseat of a seat pocket along with the itinerary and boarding pass of a government scientist who is the program manager of BioWatch, the group tasked with carrying out the biological weapon reaction tests.

Duke did not share additional steps employees can take to avoid similar types of incidents in the future, but said it served as a reminder to employees of "how careful we have to be."