Recipients of the draft "after-action" reports were told to keep them locked up after business hours and to shred them prior to discarding. They were admonished not to share their contents with anyone who lacked "an operational need-to-know."
But security surrounding the December 2017 reports suffered an embarrassing breach:
A CNN employee discovered copies of them, along with other sensitive DHS material, in the seat-back pocket of a commercial plane. The reports were accompanied by the travel itinerary and boarding pass of the government scientist in charge of BioWatch, the DHS program that conducted the anthrax drills in preparation for Super Bowl LII in Minneapolis.
The reports were based on exercises designed to evaluate the ability of public health, law enforcement and emergency management officials to engage in a coordinated response were a biological attack to be carried out in Minneapolis on Super Bowl Sunday.
The exercises identified several areas for improvement, including the problem that "some local law enforcement and emergency management agencies possess only a cursory knowledge of the BioWatch program and its mission."
CNN decided to withhold publication of this article until after the Super Bowl after government officials voiced concerns that publishing it prior to the game could jeopardize security for the event. A DHS official told CNN that areas for improvement identified in the draft reports had been addressed prior to Sunday's game and that the agency had "great confidence" in its preparedness.
"This exercise was a resounding success and was not conducted in response to any specific, credible threat of a bioterrorism attack," said Tyler Q. Houlton, an agency spokesman.
Juliette Kayyem, a former DHS official who now serves as a CNN contributor, said it was not surprising that the documents highlighted deficiencies.
She said such exercises are designed to expose gaps in planning and preparedness so that authorities "are better equipped if something bad were to happen."
Nonetheless, she said, the misplacement of the documents was "a really stupid thing."
"Who knows who else could have picked this up," she said.
"The biggest consequence 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."
In addition to requesting that CNN not publish prior to the Super Bowl, DHS officials argued that disclosure of some material contained in the draft reports could threaten national security, regardless of when it was published. Based on that concern, CNN is withholding some details contained in the documents.
The after-action reports obtained by CNN are based on a pair of exercises conducted as part of DHS's BioWatch program, which operates a nationwide aerosol detection system designed to provide an early warning of a biological attack across all levels of government.
The exercises -- one in July, the other in early November — were built around the response to an intentional anthrax release that coincides with the Super Bowl.
Among the findings was that there were "differences of opinion" over how many people had been exposed, "which led to differences of opinion on courses of action."
The reports also noted there was confusion among local health agencies about the meaning of alerts issued during the exercise and with whom information could safely be shared during an emergency.
This "made it difficult for them to assess whether their city was at risk," the documents stated, and "creates a situation where local officials are deciding on courses of action from limited points of view."
CNN was unable to verify who left the documents on the plane. The travel itinerary and boarding pass accompanying the documents was in the name of Michael V. Walter.
Walter, a microbiologist, has been the program manager of BioWatch since 2009, according to his LinkedIn profile.
"I am responsible for developing and operating a budget that has ranged up to 90 million dollars and directed a staff or more than 50 members," his profile says.
He held previous posts with the Central Intelligence Agency and Naval Surface Warfare Center and has 20 years of experience with biological warfare research.
Walter, 59, did not respond to requests for comment for this article.
A DHS official said the missing documents were the subject of an "operational review" and that "DHS does not comment on personnel matters or potential pending personnel action."
There has been a drumbeat of criticism surrounding the BioWatch program since its inception in 2003.
Multiple government reports issued over the course of more than a decade have raised questions about its cost and effectiveness.
"Since 2003, approximately $1 billion has been spent on this program," according to a 2013 memo by the House Committee on Energy and Commerce's oversight subcommittee. "After more than a decade of operation, DHS still lacks crucial data demonstrating the effectiveness of the current technology."
The report also noted differences of opinion within the government about the program.
"Several statements by DHS about the performance of the BioWatch program are disputed by other government scientists or contradicted by information obtained in this investigation," the document said.
A 2015 report by the Government Accountability Office struck a similar tone when it said "considerable uncertainty" exists about the types and sizes of biological attacks the system could detect. The report states that because DHS did not develop "performance requirements" for the program, the agency could not make informed decisions about how to upgrade it.
An agency official noted in an email to CNN that "Biodetection is one aspect of a layered approach to biodefense," and that "DHS continues to develop requirements and field enhancements to our national biodefense."