Emergency responders rely largely on wireless communications to coordinate operations at the scene, find those who are injured and rescue them from the wreckage. And ordinary citizens use mobile devices to alert loved ones to their status or whereabouts.

But what also usually happens near accident scenes like the one in Minneapolis is a disruption in cellular service because there's too much radio and network congestion. Many news outlets reported that cell phone service in the greater Minneapolis area went down, citing the fact that cell phone towers and antennae were overloaded by the sheer number of users trying to place calls.

One story reported that "Jay Reeves, 39, was one of the first people on the scene after the collapse. He tried calling 911, but all the lines were jammed."

A separate first-person account of Wednesday's events in Minneapolis detailed the cellular disruptions and related frustrations of many. "While I was out, I got a dozen or so SMSes but only one or two calls. [Only] every tenth call I tried to make went through, and half of the successful ones had problems like not hearing the other end, dropping or unusable quality," wrote Charlie Demerjian, a contributor to The Inquirer website who lives just five blocks from the bridge.

"Why? Simple overload. The infrastructure could not handle it," he continued. "Friends as far away as 10-15 miles could not place calls, the entire network was teetering on going down."

This tragic incident isn't the first time such a wireless infrastructure failure has occurred--and it likely won't be the last. Recent events such as Hurricane Katrina, the London subway bombings, the 2003 electrical blackout in the Northeast United States, and the 9/11 attacks on New York City and Washington, D.C., all led to mobile phone-service outages.

So why does this happen?

According to David Crowe, a 20-year veteran of the cell phone industry and founder of Cellular Networking Perspectives consultancy, the cause is a combination of simple overload on the cellular systems and the randomness of the events. "The problem with major tragedies is that they are completely unpredictable and often occur where large amounts of cell phone capacity are not normally required," he says.