There has never been a major security incident at a Super Bowl, America's quintessential iconic annual event, so why has U.S. law enforcement in recent years deployed every imaginable protective measure, despite the fact there has been no specific, credible threat identified?
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The short answer is that you don't know what you don't know.
And there are some moments in history that remind law enforcement that major sporting events have been the stage for terror, including the horror of the Munich Olympics in 1972, the Centennial Olympic Park bombing of 1996 in Atlanta, and the more recent Boston Marathon bombing.
Customs and border protection program manager Ronald Nunn was on the scene in Atlanta in 1996, so he knows that anything can happen anywhere.
"1996 during the Olympics in Atlanta I was a block away when the bomb went off," Customs and Border Protection program manager Ronald Nunn said. "You know, most people are running away; I ran towards it. I was first on scene commander for the first 30 minutes of that bombing."
Remembering that frightening night, he tries to prepare for every conceivable ugly scenario.
High-tech scanners are being used to search every item entering the University of Phoenix Stadium -- from fixtures and food to the costume worn by halftime performer Katy Perry, Nunn said.
"Katy Perry's stage came in [Wednesday night]," Nunn said. "We've got port-a-potties. We've got everything -- food, hot dogs, hamburgers, the NFL paraphernalia, jerseys, everything."
Officers will be wearing portable radiation detectors, and bomb-sniffing dogs will also be employed.
Secretary of Homeland Security Jeh Johnson, in an exclusive interview inside the stadium, told ABC News that security experts are on high alert, with a focus on smaller-scale and lone wolf-style attacks.
"Our challenges in Homeland Security are evolving. We have more concerns about domestic-based acts of violence, inspired by things people may see or read on the Internet," Johnson said.
While Johnson said there are no credible threats against this weekend's game, he said authorities are prepared to respond by any means necessary. That security involves a U.S. Customs Black Hawk helicopter, part of a fleet of aircraft guarding the Super Bowl from above. F-16 fighter jets will also be in the air.
Roughly 30 miles of airspace over the big game is restricted.
"There is no room for complacency," Johnson said. "Law enforcement, Homeland Security, needs to be on guard. We do not want to discourage people from coming to these types of events, but all of us in government and law enforcement need to be vigilant. We need to be on guard. Absolutely."
The Super Bowl transcends sports, drawing people who could care less about sports, but watch the game as an entertainment event.
"It's more than a football game," Johnson said. "It's an event. It's an American tradition. And so it's the focus of a lot of attention, and therefore it should be the focus of a lot of our efforts in government at public safety, law enforcement, Homeland Security, and federal state and local security. And I believe we will have a successful and safe event."