CNN reports the documents, dated December 2017, criticised the planned response to an anthrax attack on the US Bank Stadium in Minneapolis by terrorists, and made recommendations for improvement based on simulation exercises carried out to assess the authorities' ability to respond to a biological attack in a co-ordinated manner.
CNN said on its website it had delayed reporting about its discovery until after the game, at the request of federal officials.
The errant document offered a critique of how officials performed during a simulated release of the infectious disease anthrax in Minneapolis on the day of the Super Bowl. The contents should not be shared with individuals who do not have operational need-to-know. One government scientist apparently didn't do so, as a CNN employee found out on a recent commercial flight, the network reports.
A CNN employee discovered copies of them, along with other sensitive DHS material, in the seat-back pocket of a commercial plane.
CNN could not verify that it was Walter who left the docs on the plane, however.
"This exercise was a resounding success and was not conducted in response to any specific, credible threat of a bioterrorism attack", a DHS spokesperson told CNN.
Houlton declined to confirm whether any sensitive documents related to the Super Bowl were misplaced.More news: Salman Khan's 'Mujhe Ladki Mil Gayi' tweet sends internet into frenzy
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"It's shows sort of casualness about the safety and security that we're promising our state and local partners when we do these exercises and training", said the former Assistant Secretary of DHS Juliette Kayyem.
"[Leaving those documents was] a really stupid thing", Kayyem said.
"The biggest effect of this mistake", Kayyem said, "may have less to do with terrorists knowing our vulnerabilities and more to do with confidence in the Department of Homeland Security".
"In the end, confidence in the federal government at a time of crisis is what the American public deserves".
Included in the documents was the travel itinerary of a senior manager at DHS - the man in charge of the government's detection and response to a bio-terror attack. Among some of the findings listed in the documents were "differences of opinion" over how many people had been exposed.
The documents revealed that during the drills, conducted in July and November, local health agencies were confused about the meanings of alerts that were issued.